Stage 1: Remove fit, fitmultilevel, benchmark.

Also: dependency on scikit-learn is no longer needed in core.
remove modeldriven
remove DX, DP, DL, modelinstance
mst fix
fixes mst test
remove SimilarityModel2D, fixes for diff subclassing
add master channel to CI
Merge pull request #498 from nontas/mst_fix

Minimum spanning tree bug fix
Merge pull request #499 from jabooth/rmfit

spin out AAM, CLM, SDM, ATM and related code to menpofit