Folder and file naming convention

[agoscinski]

To be closer to the naming convention of sklearn some propositions how to change the naming:

subpackage naming

• pcovr -> decomposition
• selection -> feature_selection

filename convention in subpackages:

• filename.py -> _filename.py (this is not that important I guess)
• base classes of a subpackage to a _base.py

test naming convention:

• subpackage/test_filename.py (I wouldn't be strict about the class structure, one test class might cover multiple classes)

[rosecers]

In general in favor, although issues arise with sample selection.

[agoscinski]

decided to move selector to feature_selection and have a wrapper code in selection (EDIT or other way around)

[rosecers]

Addressed in #94