Sparse map potential

mapPotential() in SP returns a sparse array of active indices only. 
Fixes c++ part of https://github.com/numenta/nupic/issues/1385 

I was thinking to keep the old positional mask (rather than the indices) in comments for reference, as it gives a better visual feel for what the problem is. 

`    0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0, 0,
     0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0, 0,
     0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0, 0,
     0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0`

vs

`{6, 7, 8, 18, 19, 20, 30, 31, 32}`