Fix windows installation problem (Issue #1634)

[badpandabear]

As described in issue 1634 code added to support OpenMP was failing on windows because gcc was not installed. Through testing I found that installing gcc was not sufficient to deal with this because setup tools would always use the Microsoft Visual Studio compiler when building the extension.

It turns out the extension_helpers package, already used by PypeIt, had similar code to test for and use OpenMP that worked on windows. So I updated bspline/setup_package.py and pypeit_c_enabled to use that code instead. I also updated the installation docs to provide more information on how to install under Windows and how to enable OpenMP on Macs. I also referenced Windows Subsystem for Linux as an alternative for running under Windows.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 38.51%. Comparing base (9e19d4c) to head (46dfe13).
Report is 404 commits behind head on develop.

Files with missing lines	Patch %	Lines
pypeit/bspline/setup_package.py	0.00%	4 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1831      +/-   ##
===========================================
+ Coverage    38.49%   38.51%   +0.01%     
===========================================
  Files          207      207              
  Lines        48171    48151      -20     
===========================================
  Hits         18545    18545              
+ Misses       29626    29606      -20     

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[tepickering]

i noted a couple changes to be done, most importantly update the version of gcc from 12 to 14. or just specify the homebrew gcc specifically. everything else looks good. thanks for the quick work!

[kbwestfall]

Looks great! Thanks for the detailed update to the windows installation instructions!