Time-based refractory period

[wsphillips]

From parent issue #66

Add a cached value for the the time stamp of the last spike event for each neuron so that we can support time-based refractory periods in addition to the current simple check for positive-going threshold crossings. A spike would then occur only when all of the following conditions are met:

1. V > V_threshold
2. V_previous < V_threshold
3. t - t_lastspike > refractory_period

When a spike occurs then the associated effect should additionally reset the value of t_lastspike