grouping api

[jhamman]

This a first attempt at the grouping api I discussed in #49. It introduces a new Grouper object that supports iteration by group. I've written up a padded day-of-year grouper as a demo.

cc @dgergel, @jukent

closes #49

[dgergel]

See the padded day-of-year grouper that already exists with the same name in #28. There is good functionality in the one you have here, but it should be built off of what's already in #28 given that that was already under development, this borrows logic from that one, and that PR was already ready to be merged.

[jhamman]

@dgergel - yes, absolutely. The individual groupers are not the interesting part of this PR. The GroupedRegressor is the bit we should all take a look at to evaluate if it could work for our existing (e.g. BCSD/Zscore) or future implementations.

[jhamman]

Merging as is so we can start playing with the new api. @dgergel, I've kept the separate namespaces for groupers and grouping for now. Eventually, those will probably be merged but for now, this gives us the option to proceed incrementally.