Added earth-relative wind field calculation to base diagnostics

[lpilz]

Change Summary

This might not be the best way to implement this, but I think we should also include earth-relative wind field calculation if we already have the other diagnostics. What are your opinions?

Checklist

• [x] Unit tests for the changes exist
• [x] Tests pass on CI
• [x] Documentation reflects the changes where applicable

PS:

For me, locally, this change causes the test tests/test_accessors.py:15 test_postprocess[lambert_conformal-lambert_conformal_conic] to fail, though I think this is an unrelated bug. However, I don't know enough about cf_xarray to debug this.

The error is:

>       assert 'z' in standard_names['atmosphere_hybrid_sigma_pressure_coordinate']                                                                                                            
E       AssertionError: assert 'z' in ['z_stag']                                                                                                                                               

[lpilz]

I think I'll merge pursuant to your and @andersy005's review. Thanks, guys!