refactor adapter.py

[kamil-kaczmarek]

Problem: Currently User must from steppy.adapter import Adapter, E in order to use adapters.

Refactor so that:

• Use does not have to import E
• add Example to docstrings

Refactor is comprehensive, so that:

• correct the code
• correct tests
• correct docstrings

[kamil-kaczmarek]

@mromaniukcdl here is the task about adapter.py refactor.
Branch is ready for you: https://github.com/minerva-ml/steppy/tree/dev-adapter

[kamil-kaczmarek]

@mromaniukcdl closing this.