Document InteractiveSpikeGen

[mbeyeler]

The InteractiveSpikeGen class (tools/spike_generators/interactive_spikegen.h) needs documentation and a corresponding section in User Guide Chapter 6, possibly 6.1.4 (make "Generating Custom Spikes" the last subsection of 6.1).

This SpikeGen is similar to PeriodicSpikeGenerator: It produces spike with a certain ISI. However, it's "interactive" because it lets you update the firing rates during the simulation. It also lets you set a different firing rate per neuron.
It also lets you set a "quota" (some number of spikes) to schedule. Each scheduled spike (with appropriate ISI) decrements the quota. Once the quota reaches zero, no more spikes will be scheduled.

Maybe we could also merge this functionality with the existing PeriodicSpikeGenerator...