refactor: make offsets to convert to MJD variables

[tsutterley]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 69.47%. Comparing base (76a288a) to head (cbe4239).
Report is 1 commits behind head on main.

Files	Patch %	Lines
pyTMD/astro.py	88.23%	2 Missing ⚠️
pyTMD/compute.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #306      +/-   ##
==========================================
+ Coverage   69.34%   69.47%   +0.13%     
==========================================
  Files          39       39              
  Lines        8844     8892      +48     
  Branches     1195     1195              
==========================================
+ Hits         6133     6178      +45     
- Misses       2325     2328       +3     
  Partials      386      386              

Flag	Coverage Δ
unittests	69.47% <87.50%> (+0.13%)	⬆️

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