Refactor `find_tgt_info.py`

[mfixstsci]

A few things I would ideal like to change:

• pa = 'X': this flag is used for logic and looping. I don't know exactly what 'X' represents but I think there is a more "pythonic" solution.
• the list definition and population: https://github.com/spacetelescope/jwst_gtvt/blob/master/jwst_gtvt/find_tgt_info.py#L322
• the call to bound_angle: https://github.com/spacetelescope/jwst_gtvt/blob/master/jwst_gtvt/find_tgt_info.py#L352
• figure generation could be more pythonic: https://github.com/spacetelescope/jwst_gtvt/blob/master/jwst_gtvt/find_tgt_info.py#L416