Implemented decoded-to-nonlinearity connection

Implemented direct-to-neuron (direct-to-nonlinearity) connections.

This makes it possible to do inhibitory connections.

Also added an python notebook example script on how to make such a
connection. Although, someone has the write the description blurb for
me. I'm terrible at that.
Added unit tests direct-to-nonlinearity

- Added unit tests for direct-to-nonlinearity connections
- Renamed python notebook filename to match convention
Updated inhibitory connections

Lots of changes due to new connection system
Fix error with neuron gains being regenerated

- Fixed issue with neuron gains being regenerated if they were
  already generated by the calc_direct_connect_transform function
- Updated inhibitory_gating ipython notebook. Fixed formatting.
Updated inhibitory_gating notebook

Increased weighting on inhibitory signal to ensure complete
inhibition (due to randomness, it might always be fully inhibited for
the decoded-to-nonlinearity connection).