Sparse map potential

makes SP mapPotential() return a sparse array of active indices. 
python part.
Blocked by: https://github.com/numenta/nupic.core/pull/201
Fixes #1385 
I don't know either. Even didn't test if/how the code works without that. These lines occur already in the class with the same meaning, so I've just copied that to the new appropriate place...