perf: ability to pre-fix the BCRS coordinates

[steven-murray]

This adds the option to pre-set the _bcrs coordinates in the ERFA coord manager. This is useful if using the option update_bcrs_every=np.inf, so that the BCRS matrix is set outside the setup() (which is called inside multiprocessing tasks).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.85%. Comparing base (8b97962) to head (5c7dc96).
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #97      +/-   ##
==========================================
+ Coverage   97.82%   97.85%   +0.02%     
==========================================
  Files          21       21              
  Lines         828      838      +10     
  Branches       89       93       +4     
==========================================
+ Hits          810      820      +10     
  Misses          9        9              
  Partials        9        9              

Flag	Coverage Δ
unittests	97.25% <100.00%> (+0.03%)	⬆️

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[tyler-a-cox]

This looks good to me! Thanks @steven-murray