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 

Similar to other comment, how about this for consistency:

```
    // Binary representation:
    // 000000111000
    // 000000111000
    // 000000111000
    // 000000000000
    // 000000000000
    // 000000000000
    UInt expectedMask4[] = {
    0, 8, 9, 10, 11,
    12, 20, 21, 22, 23,
    24, 32, 33, 34, 35,
    36, 44, 45, 46, 47,
    60, 68, 69, 70, 71 };
```