Some additions for the FilterGenerator:
 - added Bessel low- and highpass-filters
 - added a filter that adds a constant group delay
 - some minor changes in the documentation
ConstantSignalGenerator, a module that generates a Signal with a constant
sample value
Test and document the behavior of the NoiseGenerator module when caching
is enabled or disabled
SignalEnvelope, a module for calculating the envelope of a Signal
ShortTimeFourierTransform, a module for calculating a short spectrum of a signal
Merge branch 'current' into gh-pages