Spontaneous activity

[abrahamnunes]

This pull request adds spontaneous activation (SA, measured in Hz) to the granule cells. It does this by adding a NetStimBox to each GC.

There are still a few things that need to be done though:

• Need to have an automated way of measuring SA in the absence of PP input, prior to a full run of the pattern separation protocol
• Need to better control the resulting firing rates for a given weight between the SA generator and its GC.