Better missing units handling

[taldcroft]

Description

This fixes a couple of issues:

• Unknown unit transform between count and HZ. Although formally these are not equivalent, for 2TLEV2RT the CXC and MSFC1949 do use those so just make it work.
• Do not raise an exception for an unknown unit transformation, just issue a loud warning. I think this was the intent of the original code.

Interface impacts

None

Testing

Unit tests

• Mac

(ska3) ➜  cheta git:(better-missing-units-handling) git rev-parse --short HEAD
1098b39
(ska3) ➜  cheta git:(better-missing-units-handling) pytest
================================================== test session starts ==================================================
platform darwin -- Python 3.13.11, pytest-9.0.2, pluggy-1.6.0
rootdir: /Users/aldcroft/git
configfile: pytest.ini
plugins: anyio-4.12.1, timeout-2.4.0
collected 196 items                                                                                                     

cheta/tests/test_comps.py ..............s.......................................................                  [ 35%]
cheta/tests/test_data_source.py ...........                                                                       [ 41%]
cheta/tests/test_fetch.py ..........................................                                              [ 62%]
cheta/tests/test_intervals.py .........................                                                           [ 75%]
cheta/tests/test_orbit.py .                                                                                       [ 76%]
cheta/tests/test_remote_access.py ......                                                                          [ 79%]
cheta/tests/test_sync.py ........                                                                                 [ 83%]
cheta/tests/test_units.py ...........                                                                             [ 88%]
cheta/tests/test_units_reversed.py ...........                                                                    [ 94%]
cheta/tests/test_utils.py ...........                                                                             [100%]

====================================== 195 passed, 1 skipped in 104.00s (0:01:44) =======================================

Independent check of unit tests by [REVIEWER NAME]

• OSX

(ska3-latest) flame:cheta jean$ pytest
================================================================ test session starts ================================================================
platform darwin -- Python 3.13.11, pytest-9.0.2, pluggy-1.6.0
rootdir: /Users/jean/git
configfile: pytest.ini
plugins: anyio-4.12.1, timeout-2.4.0
collected 196 items                                                                                                                                 

cheta/tests/test_comps.py ..............s.......................................................                                              [ 35%]
cheta/tests/test_data_source.py ...........                                                                                                   [ 41%]
cheta/tests/test_fetch.py ..........................................                                                                          [ 62%]
cheta/tests/test_intervals.py .........................                                                                                       [ 75%]
cheta/tests/test_orbit.py .                                                                                                                   [ 76%]
cheta/tests/test_remote_access.py ......                                                                                                      [ 79%]
cheta/tests/test_sync.py ........                                                                                                             [ 83%]
cheta/tests/test_units.py ...........                                                                                                         [ 88%]
cheta/tests/test_units_reversed.py ...........                                                                                                [ 94%]
cheta/tests/test_utils.py ...........                                                                                                         [100%]

================================================================= warnings summary ==================================================================
cheta/cheta/tests/test_comps.py::test_cmd_states
  /Users/jean/miniforge3/envs/ska3-latest/lib/python3.13/site-packages/setuptools_scm/git.py:427: UserWarning: git archive did not support describe output
    warnings.warn("git archive did not support describe output")

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
=============================================== 195 passed, 1 skipped, 1 warning in 151.36s (0:02:31) ===============================================
(ska3-latest) flame:cheta jean$ git rev-parse HEAD
1098b39f3b68ced1c7b4c71f5a4cab653eae361e

Functional tests

With the new unit equivalence commented out, the warning appears.

>>> from cheta import fetch, fetch_eng, fetch_sci
...
... msid = "2TLEV2RT"
... start = "2026:065:13:39:10"
... stop = "2026:065:13:40:30"
... dat_cxc = fetch.Msid(msid, start, stop)
... dat_cxc.unit
'count'

>>> dat_eng = fetch_eng.Msid(msid, start, stop)
... dat_eng.unit
/Users/aldcroft/git/cheta/cheta/units.py:309: UserWarning: 


**** WARNING ****
For MSID 2TLEV2RT the requested unit conversion from count to HZ
does not have a defined transformation function.
You may be getting incorrect results now.

PLEASE REPORT THIS to the Ska developers!

  warnings.warn(
'HZ'

>>> dat_sci = fetch_sci.Msid(msid, start, stop)
... dat_sci.unit
'count'

[jeanconn]

I was expecting a better match between maude and cxc telem fetched with cheta, but maybe this is to be expected.

[jeanconn]

Changes look good and make sense.