NF: Find the closest vertex on a sphere for an input vector.
BF + NF: Implement find_closest for Hemisphere.
Account for antipodal symmetry as appropriate for Sphere/Hemisphere.
TST: Demonstrate that this is scale-invariant.
PEP8: whitespace between functions.
Merge pull request #483 from arokem/find_closest

NF: Find the closest vertex on a sphere for an input vector.
NF: Starting to implement the SFM.
RF: Make sklearn an optional package.
NF: First pass at fitting the thing.
Starting to work on signal prediction.
Abstract out the design matrix construction. Predict.
BF: Make it work with different in/out situations.

On to test that it does something sensible (signal-wise).
TST + BF: More testing, more fixes.
TST: More tests.
Handle errors in the ODF estimation.
When signal is zero, set params to all-zeros, not nans.
TST: update the tests.
NF: Calculate the ODF from the parameters.
RF: Cache for speed.
DOC: Small fixes in documentation.
DOC: Reference