Network example

Resolves numenta/nupic.core#154

Adds the example `examples/network/network_api_demo.py` that computes identical anomaly scores to `examples/opf/clients/hotgym/anomaly/hotgym_anomaly.py` but using the Network API directly instead of the CLAModel and OPF.
> My rationale was in advocating for the committed version (always indent two spaces) - see the link below. 
> 
> https://github.com/numenta/nupic/blob/70b04e46ae18faf20d58f0b707b5b8a7204aade2/examples/network/network_api_demo.py#L34-L35

Ok that makes more sense to me.