LatticeCombiner interface

[garfieldnate]

Add a LatticeCombiner interface whose role is to combine sublattices. Then we can have the original algorithm from AM::Parallel beside the current intermediate combination one. This will make comparing them possible, but will also make switching possible if the intermediate combination one turns out to be too memory-hungry for a given data set. I plan on there being more combiners in the future as well, after reading the literature for fast set intersection.