[MRG+1] MAINT Refactor  univariate_selection module

The refactoring : 
- simplifies the class hierachy of univariate feature selection transformer (_BaseFilter, _PvalueFilter, _ScoreFilter to only _BaseFilter)
- improves and performs all parameter checks during the fit,
- fix some pep8
- set the file to ascii only (instead of utf-8)
- fix issue #3132