Broke apart a linmos parset creation

[tjgalvin]

Separated our some of the steps that go into creating the linmos parset file. Did this as for some time had incorrectly been specifying the rotation alpha variable in the parset. Some tests added to try to help make sure this does not happen.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.

Project coverage is 56.49%. Comparing base (cdab453) to head (ace1d97).

Files	Patch %	Lines
flint/coadd/linmos.py	94.44%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #163      +/-   ##
==========================================
+ Coverage   56.23%   56.49%   +0.26%     
==========================================
  Files          33       33              
  Lines        5104     5110       +6     
==========================================
+ Hits         2870     2887      +17     
+ Misses       2234     2223      -11     

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