introduce (but don't yet use) a Stopper trait
use the stopper in expand
WIP for in-memory expansion
first cut at in-memory expansion
be more defensive about not infinitely recursing
limit num reducers
limit the number of in-memory expansion steps
choice of Json vs Kryo serialization
try having a hard threshold on local splits too; up the default distributed threshold; force to reducers when expanding
simplest way to get proper numbers on the new leaves
update the README
add a note in the README about serialization
be more defensive about the possibility of being asked to expand with 0 instances
Merge pull request #8 from stripe/avi-stopper

in-memory expansion