Change API to match sklearn and update files and notebooks

Update API syntax to be in line with syntax from sklearn. For example
Nsample is now n_samples, Nphase is now n_phases, etc. Update the
notebook examples with new samples and fix issue cased by internal links.
Add Cahn-Hilliard example and optimize_n_states

Fix issue #75

Add Cahn-Hilliard example notebook. As part of the notebook examples
add optimize_n_states to explore the number of local states that can
be used in a MKSRegressionModel with a continuous microstructure. Fix
typo in Elastic_2d example. Add cahn-hilliard example to docs.