Groupby w filter update v1

@CarstVaartjes
CC @esc
This PR merges last changes from BCOLZ master official repo and esc:pxd_v3 (https://github.com/Blosc/bcolz/pull/98) and a small 'fix import ndarray'

I ran a couple of groupby functions with different parameters, seems to work fine but please keep in mind that it was not extensively tested.