Add atand function

[AdamRJensen]

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pvlib has convenience functions for calculating sine, cosine, and tangent in degrees. Similarly, it has functions for arcsine and arccosine but NOT arctangent, which seems inconsistent.

I couldn't find anywhere these functions are tested, but maybe they just aren't?

[AdamRJensen]

I will say that I wish we would have named inverse sine arcsind and not asind to stay consistent with the Numpy naming which calls the radian equivalent (which we wrap) for np.arcsin. @kandersolar do you know if this was an intentional choice?

[echedey-ls]

Would atan2d be a good addition to this PR too? Or arctan2, following last comment.

[AdamRJensen]

Would atan2d be a good addition to this PR too? Or arctan2, following last comment.

Certainly not a bad idea - I think I'll hold off adding it until I need it though :P

[kandersolar]

asind is among the oldest bits of pvlib python: e74644b#diff-0122268ff3ca7c61f9b9be534e547dbdfff3a7509577e704e456f06091908dfbR250-R252

I presume the choice of name was motivated by convenience when translating to python from MATLAB (which also uses asind).

[echedey-ls]

Would atan2d be a good addition to this PR too? Or arctan2, following last comment.

Certainly not a bad idea - I think I'll hold off adding it until I need it though :P

For future reference, search of np.degrees(np.arctan2( among all pvlib module raises 16 results.

Valid point anyway.

[kandersolar]

Since normally I probably wouldn't be in favor of adding functions to pvlib.tools without it being used somewhere in pvlib, I'll point out that we expect to use atand in an upcoming PR.

Thanks @AdamRJensen