WIP: Started implementing cross-validation from model predictions.
ENH: A bit more work on cross-validation.
BF: The order of the output needs to be like the order of the data...

Also: some more flexibility on the model API that is accepted.
PEP8

Before I start mucking about here.
PEP8
PEP8
BF: Predict properly, by separating b0s and diffusion-weighted samples.
TST: This test currently fails.

Fix to follow immediately :-)
BF: Fix rotation matrix calculation with a new implementation.
WIP: Work towards xval prediction in csdeconv.
DOC: optional arguments and their default values.