Perf: Densify prevActiveCells rather than binary searching
Perf: Densify prevActiveCells rather than binary searching

I learned while changing the SparseMatrix that this strategy is much
faster, despite using more memory.

Hotgym runtime:
Before: 2.04 seconds
After: 0.90 seconds

ywcui1990/nupic.core,numenta/htmresearch-core,ywcui1990/nupic.core,breznak/nupic.core,rcrowder/nupic.core,numenta/htmresearch-core,rhyolight/nupic.core,numenta/nupic.core,rhyolight/nupic.core,rcrowder/nupic.core,numenta/nupic.core,neuroidss/nupic.core,scottpurdy/nupic.core,lscheinkman/nupic.core,subutai/nupic.core,rcrowder/nupic.core,EricSB/nupic.core,ywcui1990/nupic.core,EricSB/nupic.core,rhyolight/nupic.core,lscheinkman/nupic.core,lscheinkman/nupic.core,EricSB/nupic.core,subutai/nupic.core,numenta/nupic.core,breznak/nupic.core,ywcui1990/nupic.core,breznak/nupic.core,EricSB/nupic.core,neuroidss/nupic.core,subutai/nupic.core,neuroidss/nupic.core,breznak/nupic.core,rcrowder/nupic.core,lscheinkman/nupic.core,scottpurdy/nupic.core,numenta/nupic.core,rhyolight/nupic.core,scottpurdy/nupic.core,numenta/htmresearch-core,neuroidss/nupic.core,scottpurdy/nupic.core,numenta/htmresearch-core,subutai/nupic.core