Change colours of high/low voltage in simulation playback

[pgleeson]

These colours can be changed in the source code by altering these lines: https://github.com/NeuralEnsemble/neuroConstruct/blob/master/src/ucl/physiol/neuroconstruct/gui/SimulationRerunFrame.java#L1554

but it would be nice to update the gui:

so this can be set & saved by a user