initial commit of SpikePlot.cpp and SpikePlot.h which define a general spike plotting object that can completely replace ElectrodePlot, TetrodePlot, StereotrodePlot. SpikePlot objects can be added to the canvas and render fine, however, eventually the Tetrode variant of the SpikePlot causes a segfault, probably due to the way that spikes are copied around in the SpikeDisplayCanvas.