refactor polygon to models module

[wsavran]

moved polygon class to models module.

[codecov-commenter]

Codecov Report

Merging #135 (600e470) into master (e0d0065) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #135      +/-   ##
==========================================
+ Coverage   56.97%   57.00%   +0.02%     
==========================================
  Files          19       19              
  Lines        3147     3149       +2     
  Branches      455      455              
==========================================
+ Hits         1793     1795       +2     
  Misses       1243     1243              
  Partials      111      111              

Impacted Files	Coverage Δ
csep/core/catalogs.py	60.00% <ø> (-0.08%)	⬇️
csep/core/forecasts.py	55.68% <100.00%> (+0.16%)	⬆️
csep/core/regions.py	52.20% <100.00%> (-1.46%)	⬇️
csep/models.py	42.16% <100.00%> (+7.05%)	⬆️

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