Create SP Mixin #64

@chetan51 @subutai 

I have created a mixin monitor class for the spatial pooler. Currently the following metrics are collected
1. Indices of active column
2. Active duty cycle of each column
3. Total number of connections.

Let me know if more metrics are desired to be collected.


Is this already computed by the spatial pooler code? If so, can we just record the computed values on each iteration in a trace?

If not, does it make sense to compute this as a trace, rather than as a summary over the entire history?