FEATURES.md and docs update for patch

[jajhall]

Initialised FEATURES.md, updated docs/src/options/definitions.md and corrected docs/src/interfaces/python/model-py.md

[jajhall]

@galabovaa and @filikat: please check the "Build changes" section, and add the "Code changes" section

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.34%. Comparing base (48487c6) to head (af1592f).
⚠️ Report is 29 commits behind head on latest.

Additional details and impacted files

@@            Coverage Diff             @@
##           latest    #2837      +/-   ##
==========================================
+ Coverage   80.27%   80.34%   +0.06%     
==========================================
  Files         348      348              
  Lines       86212    86216       +4     
==========================================
+ Hits        69210    69267      +57     
+ Misses      17002    16949      -53     

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