Incorporate multiprocessing into fftvis

[tyler-a-cox]

This PR adds parallel processing to fftvis via the multiprocessing module.

[codecov]

Codecov Report

Attention: Patch coverage is 85.06494% with 23 lines in your changes missing coverage. Please review.

Project coverage is 85.18%. Comparing base (c9a83f6) to head (2785992).

Files with missing lines	Patch %	Lines
fftvis/simulate.py	89.16%	6 Missing and 7 partials ⚠️
fftvis/utils.py	66.66%	6 Missing and 3 partials ⚠️
fftvis/beams.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
- Coverage   91.53%   85.18%   -6.35%     
==========================================
  Files           4        4              
  Lines         189      243      +54     
  Branches       44       42       -2     
==========================================
+ Hits          173      207      +34     
- Misses          8       20      +12     
- Partials        8       16       +8     

Flag	Coverage Δ
unittests	85.18% <85.06%> (-6.35%)	⬇️

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