Adding logic to handle prop that end in "ref(s)"

[ryantxu1]

Resolves issue #46

If substring "ref(s)" in the property name, it generates a ListProperty with a subclass of ReferenceProperty, solving the ValueError. It detects the valid_type by looking at the spec's stix-type

[codecov-commenter]

Codecov Report

Merging #47 (4d0922d) into main (04df298) will decrease coverage by 0.05%.
Report is 2 commits behind head on main.
The diff coverage is 77.77%.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
- Coverage   90.54%   90.50%   -0.05%     
==========================================
  Files          35       35              
  Lines        3470     3474       +4     
==========================================
+ Hits         3142     3144       +2     
- Misses        328      330       +2     

Files	Coverage Δ
stix2generator/stixcustom.py	88.70% <77.77%> (-2.67%)	⬇️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[ejratl]

Thanks for the bug report and the PR! The build failure is caused by a style issue:

./stix2generator/stixcustom.py:75:1: W293 blank line contains whitespace