I'm starting to implement formulas from the IAU 2015 report:
https://astropedia.astrogeology.usgs.gov/download/Docs/WGCCRE/WGCCRE2015reprint.pdf
This is a work in progress. The unit test is partially finished.
The C function Astronomy_RotationAxis() works only for the Sun and Mercury.
I want to also return a rotation matrix that reports the rotating
frame in a way more suited to graphics work. I will add this to
the type astro_axis_t later.
I forgot that my build process automatically updates
copyright years when the current year changes.
My Travis CI unit tests verify that there are no local
changes after running all the tests.
That test failed because the update_copyrights.py changed
all the "2019-2020" to "2019-2021".
