Fix supertype of RFE models.

[ablaom]

Closes #34

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.94%. Comparing base (1970915) to head (8840a55).
⚠️ Report is 3 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev      #35   +/-   ##
=======================================
  Coverage   92.94%   92.94%           
=======================================
  Files           4        4           
  Lines         156      156           
=======================================
  Hits          145      145           
  Misses         11       11           

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