Default Union Pooler excite and decay functions and unittests

[ryanjmccall]

The Union Pooler by default does not decay its pooling activation and uses the simplest linear excitation function to positively update pooling activation. More sophisticated functions have been suggested, such as an exponential decay function and a logistic excite function. Recently, a step function was suggested for the excitation. Implementation of exponential decay and logistic excite has begun, but are unfinished and require complete unittesting. See https://github.com/numenta/nupic.research/tree/master/union_pooling/union_pooling/activation

[ryanjmccall]

@ywcui1990 FYI, as I wrote above, the current default decay in the Union Pooler is no decay and the current excite is linear. I'll let you take it from here =)

[ywcui1990]

@rmccall84
Sounds good. I have implemented and used the logistic excite function and exponential decay function, and will write the unit tests for them.