Deactivate categorical variables

[adelavega]

Closes #728

• Deactivates categorical variables by default
• Adds active as kwargs to PredictorList resource/API route

This should not change how pyNS or the frontend client interactive with the API.

[codecov-io]

Codecov Report

Merging #731 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #731      +/-   ##
==========================================
+ Coverage   85.87%   85.87%   +<.01%     
==========================================
  Files          67       67              
  Lines        3022     3023       +1     
==========================================
+ Hits         2595     2596       +1     
  Misses        427      427

Impacted Files	Coverage Δ
neuroscout/resources/predictor.py	89.02% <100%> (+0.13%)	⬆️

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