ECAP with constant modulation degree but different SAM-modulation freqs added.

Added a button in ECAP-section of the gui to reproduce figure 7.8 from Volker
Hamachers dissertation (page 123).
[Bug fix] Correct Return value

Return value corrected. The old one was never returned
Feature Instantaneous Frequency Response of Auditory Nerve (IFRAN) added

Added the feature to plot the auditory nerve response time-domain pattern of a
given (mono) .wav-File. Thus you are able to see the spectro-temporal
coding of the auditory nerve using a simulation of cochlea implant processing.
This spectro-temporal pattern can then be compared to simulations of normal-hearing
processing.
The numbers of the auditory nerve fibers can be adjusted
in the parameter-File IFRAN_reproduction.m. In the same file you will also have
the opportunity to change the best frequencies of the auditory nerve fibers. Use
greenwood_place_to_freq.m to calculate which place on the basiliar membrane
correspondents to which frequency.  The reference for the resulting plot can be
found in:
Guy J. Brown, Tim Jürgens, Ray Meddis, Matthew Roberston, Nicholas R.
Clark (2011), "The representation of speech in a nonlinear auditory model:
time-domain analysis of simulated auditory-nerve firing patterns", Interspeech
2011