Support hybrid level coefficients

[sandorkertesz]

Description

This PR adds the following keys/methods to vertical. These keys only have values for parametric level types, to date "hybrid" is the only one.

• number_of_levels
• coefficients
• coefficient_names
• parametric

Key coefficients can be set (and encoded into GRIB2) the others are read-only.

import earthkit.data as ekd
fl = ekd.from_source("file", "ml_data.grib").to_fieldlist()
f = fl[0]

f.vertical.number_of_levels() # 137
f.vertical.coefficients() # tuple of A and B coeff
f.vertical.coefficient_names() # ("A", "B")
f.vertical.parametric() # True

TODO:

• documentation
• examples
• writing to GRIB1 (if needed)
• serialisation

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.52941% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.71%. Comparing base (7482120) to head (2367baa).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
tests/grib/test_grib_set_vertical.py	96.82%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1016      +/-   ##
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+ Coverage    81.57%   81.71%   +0.14%     
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  Files          236      236              
  Lines        15803    15935     +132     
  Branches       763      768       +5     
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+ Hits         12892    13022     +130     
- Misses        2686     2687       +1     
- Partials       225      226       +1     

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