Create metrics for evaluating Layer 4 sensorimotor inference

Fixes https://github.com/numenta/nupic.research/issues/13.
Builds on https://github.com/numenta/nupic.research/pull/30.
Depends on https://github.com/numenta/nupic/pull/1346.

cc: @subutai 
This is a different metric than you proposed earlier. It is possible to have a low value here (say 2) and have a poorly trained system.  On the other hand, it is possible to have higher values (say 4 or 5) and have a well trained system.