Separate Stimulus concept from base Timeseries

[stuart-knock]

Currently, stimuli or driving time-series for a simulation are both derived from Timeseries as a base-class and contained within a Timeseries class... this mixes two concepts, leading to duplicated unused members and generally making the code less clear.

The Timeseries class should serve only the purpose of being the base-class for the various specific time-series classes, eg Sine, PulseRect, PulseSigmoid, White, etc. A second class, called, for example, Stimulus, should be extracted to serve the purpose of combining multiple Timeseries for injection into the simulation, that is it should have Timeseries members but not inherit from Timeseries.