TST: Implemented tests for new interface to linear solvers.

Also: cleaned up some of the imports:

    import foo.bar as bar

is prefered to:

    from foo.bar import this, that, the_other
NF: Uniform sklearn-like interface to linear solvers.
TST + BF: Test and improve the NNLS dipy.core.optimize interface.

Specifically, in the context of sfm