Importing ants shouldn't change iris coordinate systems

[hdyson]

🐜 Bug Report

Importing ants adds an as_ants_crs method to iris coordinate systems.

How to reproduce

Step by step guide to reproduce the behaviour:

>>> import iris
>>> import iris.coord_systems
>>> iris_crs = dir(iris.coord_systems.CoordSystem)
>>> import ants
>>> ants_crs = dir(iris.coord_systems.CoordSystem)
>>> diff = set(ants_crs) - set(iris_crs)
>>> diff
{'as_ants_crs'}

Version

This bug exists in ANTS head of main

Additional Context

The solution here is probably to:

1. Add a function as_ants_crs somewhere in ANTS which performs a similar mapping as the existing method.
2. Replace usages of the method with the function.
3. Remove the method.