Add LearningCurveSplitter class for learning curve evaluation

- Add LearningCurveSplitter to splitters.py that implements sklearn's
  BaseCrossValidator interface for learning curve evaluation
- Support 'ratio' and 'per_class' data subsetting policies
- Support variable number of permutations per data size
- Update WithinSessionSplitter to expose inner splitter metadata
- Update WithinSessionEvaluation to detect and handle LearningCurveSplitter
- Add data_size and permutation columns to results when using learning curves
- Add comprehensive tests for LearningCurveSplitter
- Verified results match old _evaluate_learning_curve implementation exactly

Author: bruAristimunha

moabb/evaluations/__init__.py

@@ -12,6 +12,7 @@
 from .splitters import (
     CrossSessionSplitter,
     CrossSubjectSplitter,
+    LearningCurveSplitter,
     WithinSessionSplitter,
     WithinSubjectSplitter,
 )