[MRG] Implement CircularValidation as a scorer

[YanisLalou]

Fixes #111
Paper: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4803844&tag=1
Process described in part [5] and [6].

Small thing not in the paper but implemented:

• If the estimator predicts always the same value, then we assume that the backward_estimator will also predicts always the same value.

--> Why? Because for some estimators, if you train them with np.unique(y) = 1 you get ValueErrors (happens with SVC).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.85%. Comparing base (6d59191) to head (6360626).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #124      +/-   ##
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+ Coverage   91.72%   91.85%   +0.12%     
==========================================
  Files          43       43              
  Lines        3348     3400      +52     
==========================================
+ Hits         3071     3123      +52     
  Misses        277      277              

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[rflamary]

few comments