Modules for calculating the mean values and the variance values for the
channels of Signals and Spectrums.
  SignalMean, SpectrumMean, SignalVariance, SpectrumVariance
Corrected the name of ShortFourierTransform (it was ShortTimeFourierTransform)
and fixed a bug that had occured when executed with Python3.
This patch reduces the threading issues in the GUI slightly.
Remove one assumed behavior from the PythonAssumptions unit test for better
compatibility with PyPy.
TestQuestionnaire, a class for unit tests that ask the user about the
behavior of the program under test.
Many improvements for the plotting system.
API-Changes:
 - The Tiled...PlotPanel-classes have been removed, because a plot's layout
   can now be changed dynamically.

Improvements:
 - A framework for implementing automated downsampling of the plotted data
   (implementations of downsamplers are still missing).
 - Dynamic switching between combined and tiled plots.
 - Automatic updates of the other plots, when one plot is panned or zoomed.
 - Plots are destroyed and rebuilt less often.
 - Some improvements in the log-scaling of the x and y axis.
 - Interactive unit tests for the PlotPanel classes.
Automatic skipping of certain unit tests, when the required libraries are missing.
IntegrateSignal, a module for calculating the integral of a Signal
Fix two typos in the documentation