Add ability to calculate BJD from a single coordinate

[mwcraig]

This PR fixes #444 by doing two things:

1. No longer calculating BJD on creation
2. Adding an argument that allows the coordinates for the correction to be set.

I started out by not doing 1 -- that is, by adding a new parameter to PhotometryData.__init__ but realized that meant, eventually, changing or adding a photometry setting, which I wasn't too enthusiastic about, so I went this route instead.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.33%. Comparing base (31f01ab) to head (41bddcc).
Report is 79 commits behind head on main.

Files with missing lines	Patch %	Lines
stellarphot/core.py	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #461      +/-   ##
==========================================
+ Coverage   80.30%   80.33%   +0.03%     
==========================================
  Files          30       30              
  Lines        3945     3947       +2     
==========================================
+ Hits         3168     3171       +3     
+ Misses        777      776       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[JuanCab]

Looks good!

[JuanCab]

Why the double backticks?

[mwcraig]

I think single backticks is supposed to be a reference and double means format it as code. Not sure why that wasn't an error before, but maybe it was pointing to the attribute?