PERF: speed up where operations when splitting blocks (GH3733)#3736

Merged

jreback merged 2 commits intopandas-dev:masterfrom

jreback:mask_slow

Jun 2, 2013

Commits on Jun 1, 2013

PERF: (GH3733) where block splitting now done in at most 2 blocks rather than
jreback
committed
BUG: _interleave now returns an lcd of int64 or 2 x the intsize if
jreback
committed