Combine Headstage64ElectricalStimulator's Enable and PowerEnable properties

[jonnew]

There are de-facto two parallel enables for Headstage64ElectricalStimulator.

1. Enable: a logical enable that controls the state machine that generates stimulus sequences
2. PowerEnable: A 1 bit signal sent to the high compliance voltage power supplies.

These should be combined into a single StimulatorEnable property for this library. Currently, if Enable is true but PowerEnable is false, a stim train will be generated and applied to the analog current source, but this will not be properly powered resulting in undefined behavior at the stimulus pin. Thus if the user wants to enable the stimulator, the power should be turned on. It should be noted in the documentation that it takes some time for the power supply to stabilize, so stimuli cannot be properly delivered directly after enabling. Ill take a measurement on how long this takes and append to this issue.