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[bot]]

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 3 commits behind head on main.

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

Additional details and impacted files

```diff @@ 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 ```

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