Replaced the algorithm from helper.binomial_coefficient with one that is
much faster for high input numbers.
Moved some of the checks in SplitSignal/SplitSpectrum for out-of-bounds
channel indices from the setter method to the getter method.
New features for the SignalFile/SpectrumFile classes:
 - GetLength and GetSamplingRate/GetResolution methods
 - the capability to read itaAudio files from the ITA-Toolbox