Add function to derive the az el of planets

[tterasaki]

To visualize when boresight hits the planet, I want a function to get the trajectory of planet.

[mhasself]

Thanks -- looks useful. However, please eliminate the code duplication. The code in get_source_pos is carefully written, and when you copy-paste that elsewhere you are setting us up for divergence, and different bugs and their fixes ending up in two places instead of one.

Also ... in the future, please do not include bulk reformatting along with content changes.
Bulk reformatting should be avoided unless the situation is really bad. When reformatting is deemed necessary, it should be in a commit by itself, and probably in a PR by itself. That will help people navigate PRs and inspect the commit history in an efficient way.

[tterasaki]

Thank you for your thoughtful comments. Yeah you are rights.
Now I made a helper function _get_astrometric which is used by both get_source_pos and get_source_azel.

[mhasself]

Thanks, just fix the docstring where noted and then it's ok to merge.