assay.py implemented get_or_create for assay_prop

[mmudado]

This pull request addresses issue #...

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[codecov-commenter]

Codecov Report

Merging #306 (f77ce93) into master (0c23682) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #306   +/-   ##
=======================================
  Coverage   69.25%   69.25%           
=======================================
  Files          29       29           
  Lines        3968     3968           
  Branches      226      226           
=======================================
  Hits         2748     2748           
  Misses       1161     1161           
  Partials       59       59           

Impacted Files	Coverage Δ
machado/loaders/assay.py	76.47% <100.00%> (ø)

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