Update implementation of classification module + check before quantile calculation

[aagoumbala]

Description

Grouping of score calculations in the fit
Changing attribute name of n_samples_in_train_ by n_samples_in_val_
Adding check if 1/alpha < number of samples of scores before quantile calculation

Fixes #(issue)

Please check options that are relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

tests added in test_utils.py

Checklist:

• I have read the contributing guidelines
• I have updated the HISTORY.rst and AUTHORS.rst files
• Linting passes successfully : flake8 . --exclude=doc
• Typing passes successfully : mypy mapie examples --strict
• Unit tests pass successfully : pytest -vs --doctest-modules mapie
• Coverage is 100% : pytest -vs --doctest-modules --cov-branch --cov=mapie --pyargs mapie
• Documentation builds successfully : cd doc; make clean; make html

[codecov-commenter]

Codecov Report

Merging #84 (71e75a4) into dev (2241a66) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               dev       #84   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           11        11           
  Lines          897       926   +29     
  Branches        59        64    +5     
=========================================
+ Hits           897       926   +29     

Impacted Files	Coverage Δ
mapie/classification.py	100.00% <100.00%> (ø)
mapie/tests/test_classification.py	100.00% <100.00%> (ø)
mapie/tests/test_utils.py	100.00% <100.00%> (ø)
mapie/utils.py	100.00% <100.00%> (ø)

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