This fixes a segfault that happened when no electrode was selected on a spike detector (e.g., with no electrodes added); getSelectedItemIndex returns -1 in this case, which was causing an array bounds segfault in drawElectrodeButtons.

updated the SignalGenerator to generate spikes of varying amplitudes and at a lower rate. Minor incremental changes were made to other files

Signal Generator now generates fake action potentials!!! The phase dial can be used to scale up and down noise on the spikes

Altered the default settings of SignalGeneratorEditor and SpikeDetectorEditor to reflect the desired values for debugging the spike viewers

Channel selector buttons now can be used to toggle audio monitoring
for individual channel. One minor bug: the array of channels to monitor
is not updated when the graph is rebuilt. If new processors are added,
the mapping might be wrong.

Through the SpikeDetector Editor, it's now possible to change the channel
mapping for each electrode and adjust thresholds. It's not yet possible
to inactivate particular electrodes or change parameters over multiple
electrodes at once; these will both be necessary functions in the future.