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

• Unit tests for the changes exist
• Tests pass on CI
• 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']                                                                                                                                               

[jthielen]

Mostly looks good! I initially thought that grid-relative/earth-relative conversion would be the kind of general-purpose thing to leave to MetPy to handle, but likewise to geopotential to geopotential height, this looks to have enough WRF-specific nuance to justify handling here.

A couple small suggestions:

• Update the documentation of postprocess's Parameters to account for these new fields and that unlike the other diagnostic variables, the grid-relative winds are not dropped when drop_diagnostic_variable_components is true.
• Use expected input type for _destag_variable (see suggestion below)

[jthielen]

Looks good to me! I'll hold off merging in case you wanted further reviews from anyone else or to iterate on this in PR form still. That being said, if you want to get this in for v0.0.2 and build tutorial/blog post components around it, feel free to merge.

[lpilz]

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