Array shifting bug fix

[cassidymwagner]

This PR will fix a bug that shifted the arrays in an inconsistent direction, resulting in flipped signs for some of the structure functions. I'm also including updated tests (WIP) that evaluate the code against analytical solutions, which will be a more robust method of testing.

TODO:

• Lat-lon tests
• Periodic domain tests
  • LL tests
  • LLL tests:
    • some work in progress

[codecov]

Codecov Report

Attention: Patch coverage is 99.08257% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.58%. Comparing base (5a5201f) to head (ff05488).

Files	Patch %	Lines
fluidsf/generate_structure_functions.py	98.52%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
+ Coverage   99.16%   99.58%   +0.42%     
==========================================
  Files           8        8              
  Lines         239      242       +3     
==========================================
+ Hits          237      241       +4     
+ Misses          2        1       -1     

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