analysis.calculus directional_names documentation

[trexfeathers]

calculus.curl expects input cubes to begin with their directional name followed by whitespace/underscore - achieved via calculus.spatial_vectors_with_phenom_name. This behaviour should be documented.

The list of accepted directional names should also be documented. This is currently held in the directional_names set within spatial_vectors_with_phenom_name, which is a private method. To expose the list for documentation directional_names needs to become global, and to get Sphinx to pick up the values and any lines of text I think we would need to use Sphinx autodata. May need some sphinx config changes but not sure yet.

This is something I will need to learn more about when I have some time, so just documenting in an issue for now. Any better ideas for how to document directional_names will be welcomed.

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