Building of Ensemble Classifier from MapieClassifier

[BaptisteCalot]

Description

• Migration of some code from classification.py into estimator_classifier.py
• Adapt unit tests

Fixes #430

Type of change

New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Adapt existing unit tests to the new class and check that everything still works.

Checklist

• I have read the contributing guidelines
• I have updated the HISTORY.rst and AUTHORS.rst files
• Linting passes successfully : make lint
• Typing passes successfully : make type-check
• Unit tests pass successfully : make tests
• Coverage is 100% : make coverage
• Documentation builds successfully : make doc

[LacombeLouis]

Hey @thibaultcordier,
Great PR, thank you! Really clarifies the structure of MapieClassifier as it should have been.
Thank you!

[LacombeLouis]

Hey @thibaultcordier,
Just a last small change, be careful when changing tests, we don't want to reduce the generality of the tests performed!