Add plotting functions to sEPSM.
Add plotting functions to sEPSM.

Added function to plot an SNRenv matrix or excitation points coming from
an sEPSM simulation.
Also added a function to plot the filtered envelopes, although there
isn't a way right now to access those envelopes because the modulation
filtering is done _inside_ the SNRenv computation.

achabotl/pambox