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 

How about comment on the line before for consistency, and if this is to visualize it then no need for the commas, etc:

```
// 111100000001
UInt expectedMask3[12] = {0, 1, 2, 3, 11};
```