Refactor microstructure basis functions to make OO.

Address #75

New basis classes with common interface. The MKSRegressionModel now
takes an instance of a basis class and then uses it to discretize the
microstructure property field.
Remove doctest hack as Sfepy verbosity is fixed

Remove `sfepy...` from doctests.
Fix API docs for new basis classes

Address #75

Fix API docs for new bases classes for consistency and HTML
rendering. Move some test to more sensible locations to document
better.
Update notebook examples and documentation

Address issue #75

Update elastic_2D, elastic_2D_Multiphase, elastic_3D, cahn_hilliard,
and cahn_hilliard_Legendre notes with new api.

Update tools with new api.