Fix mrmr working with categoricals

[alex-hse-repository]

Before submitting (must do checklist)

• [ ] Did you read the contribution guide?
• [ ] Did you update the docs? We use Numpy format for all the methods and classes.
• [ ] Did you write any new necessary tests?
• [ ] Did you update the CHANGELOG?

Proposed Changes

• Cast all categoricals to floats during redundancy computation in mrmr.

Closing issues

closes #1310

[codecov-commenter]

Codecov Report

Merging #1311 (2911e30) into master (e486a24) will decrease coverage by 32.17%. The diff coverage is 0.00%.

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@@             Coverage Diff             @@
##           master    #1311       +/-   ##
===========================================
- Coverage   88.20%   56.03%   -32.17%     
===========================================
  Files         186      197       +11     
  Lines       11807    12126      +319     
===========================================
- Hits        10414     6795     -3619     
- Misses       1393     5331     +3938     

Impacted Files	Coverage Δ
etna/analysis/feature_selection/mrmr_selection.py	26.66% <0.00%> (-73.34%)	:arrow_down:

... and 134 files with indirect coverage changes

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