Adds a warning whenever a _mesh is different for a field when added to a FieldSet, or during Fieldset creation

[wyatt-fluidnumerics]

Description

Currently, mixing "flat" and "spherical" meshes in a FieldSet is allowed without warning. This can happen without intention whenever a field is created with the default mesh, which may not be the same mesh as underlies the rest of the FieldSet. This PR adds a function _warn_if_fields_use_different_meshes(fields: Iterable[Field | VectorField]) in fieldset.py which produces a FieldSetWarning if fields in the input iterable have different _mesh in their underlying grid. A call to the function is also added at the end of FieldSet.__init__ and FieldSet.add_field

Checklist

• Closes Throw warning if _mesh is not the same for all Fields when creating FieldSet #2190
• This PR targets the correct branch (main for normal development, v3-support for v3 support)

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[wyatt-fluidnumerics]

It is unclear to me whether a call to _warn_if_fields_use_different_meshes should also be added at the end of add_constant_field. My understanding is the underlying mesh of a constant field changes the effect on particles by prescribing m/s or deg/s for "flat" and "spherical" respectively. In tests/test_fieldset.py constant fields with different underlying meshes are used a number of times (notably in fieldset_two_models).

[erikvansebille]

Thanks @wyatt-fluidnumerics!

It is unclear to me whether a call to _warn_if_fields_use_different_meshes should also be added at the end of add_constant_field.

Yes, I would also add this test when adding constant fields. And any errors in the test suite should will then have to be patched?

[wyatt-fluidnumerics]

I added the check to add_constant_field and revised the tests to use the correct mesh.

[erikvansebille]

Thanks @wyatt-fluidnumerics. Happy for this to be merged!