Add a warning that the helper.differentiate function is faulty
Bugfix: Connecting two connectors announces a value change now.
New differentiation functions "differentiate_fft" and "differentiate_spline".
The helper.math.py file is renamed to helper.sumpfmath.py to avoid confusion
with the standard math module when importing.

API change: the helper.differentiate function uses a different algorithm now
The differentiation function for the DifferentiateSignal class is now
selectable.
Improved unittest for proper documentation of the signal processing classes.
Changes to the FilterGenerator class
 - the FilterGenerator only accepts one filter function now
 - the filter function can be set with the constructor
 - new filter functions CONSTANT and DERIVATIVE
 - filter functions can now calculate the transfer function in dependency
   of the output spectrum's length and resolution

API changes: the AddFilter method has been replaced by the SetFilter method
which is no longer a MultiInput. The RemoveFilter and ReplaceFilter methods
have been removed.
The MultiInputData class has a Clear method now.
The MergeSignals and MergeSpectrums classes can now take mergeable data
sets as a constructor argument.
Improvements to the RecordTransferFunction example:
 - bugfix for an error that occured, when the hardware's sampling rate was
   not the same as SuMPF's default sampling rate.
 - bugfix for the wrong labeling of kept measurement data.
 - shorter signal durations during the setup of the signal processing chain
   to speed up the start of the program.
 - the examples.RecordTransferFunction function now imports the main.py
   file, which reduces duplicate code.
Improvements to the ChannelDataProperties class.
Bugfix: The soundcard IO classes raised a RuntimeError instead of a
ValueError, when the input Signal had the wrong sampling rate.
Moved the compatibility checks of the ConcatenateSignals class to its
GetOutput method.
Improved error messages for the signal and spectrum algebra classes.
The NoiseGenerator class now accepts a seed for its Random instance in the
constructor.
Change version number in setup.py to 0.9