fast rewrite of Grid Interpolator

This version selects whole columns for removal instead of resampling
grid.  There are many ways we could select columns for removal that may
be more ideal, but I chose to leave it to chance.  The random number
generator used is deterministic across platform so no matter what we'll
always get a consistent set of keep columns for any given grid size.
Deterministic RNG was made cross platform deterministic by rehashing the
same seed with MD5 over and over, taking the first few bytes for rng
generation.