Allow non-periodic waveforms in Rhs2116StimulusSequenceDialog

[bparks13]

Currently in the Rhs2116StimulusSequenceDialog the only option to add a waveform is to apply a periodic waveform on a channel, overriding the existing stimulus if it exists.

The hardware allows for a much more flexible stimulation schedule, including non-periodic pulses. This should be implemented as a way to add pulses to the end of an existing waveform so that two (or more) pulse trains with different pulse widths and inter-stimulus intervals can be included on the same channel.

To accomplish this, the main library will need to be updated to decouple the delta table from the anodic / cathodic steps and widths. Currently, the delta table is built based off of the assumption of periodic waveforms, but if it is extended to allow for arbitrary periodicity then the Design library can simply read these waveforms and plot as-is. Additionally, the functionality to add non-periodic waveforms to existing waveforms would need to be implemented from the main library as well so that it can be easily extended to the GUI.