Updated the documentation for developers.
Changed the hide-menu in the plot's toolbar to a more intuitive show-menu.
The RecordTransferFunction example stores its settings in a config file now.
Bugfix: The MergeSignals and MergeSpectrums classes could crash when only
empty datasets were added.
Streamlined the overloaded algebra operators in the Signal and Spectrum
classes:
- now it is required that both data sets for the calculation have the same
  number of channels.
- dividing a Signal or Spectrum by a scalar value is possible now.

API-Change: When adding, subtracting, multiplying or dividing Signals or
Spectrums, the two data sets must also have the same number of channels now
Streamlined the algebra classes for Signals and Spectrums:
- now it is required that both data sets for the calculation have the same
  number of channels.
- Simplify the implicit behavior when the input data sets are not
  compatible (e.g. different sampling rate)
- reduce code by using the overloaded operators of the Signal and Spectrum
  classes.

API-Change: When adding, subtracting, multiplying or dividing Signals or
Spectrums, the two data sets must also have the same number of channels now
The ProgressIndicator classes no longer expect a method as a mandatory
constructor argument.
Allow other iterable types than lists to specify multiple observers of an
InputConnector.
New function set_multiple_values that can be used to call multiple setter
methods in a processing chain without triggering a recalculation after each
call.
Bugfix for the gui.Gauge class
When Signals or Spectrums are saved to NumPy's npz format, the files are
now compressed.

Loading old files with this updated version of SuMPF still works and
loading compressed files with older versions of SuMPF works as well, as
long as the underlying version of NumPy supports compressed npz files.
Bugfix: the LineManager classes did not update the label when reusing an
existing line in the plot.
Simplifying the code of the RecordTransferFunction example by using the
set_multiple_values function.
Adding a gauge that displays the current status of the calculations.
Allow setting the merge strategy and the length conflict resolution strategy
in the MergeSignals and MergeSpectrums classes' constructors.
Commandline parameters and makefile targets for the unittests.
Fixed a grammar error in the documentation for the ParentConfig class.
New classes for loudspeaker calculations:
 - ThieleSmallParameters, a data class for the parametric description of a
   loudspeaker
 - ThieleSmallParameterInterpretation, a class for calculating loudspeaker
   parameters from other parameters
 - ThieleSmallParameterAuralizationLinear, a class for calculating the
   output signal of a loudspeaker that is described by its parameters
 - ThieleSmallParameterAuralizationNonlinear, a class for calculating the
   output signal of a loudspeaker that is described by parameters which
   change in dependency of the loudspeakers state
A new Makefile target for running all unit tests except for the incomplete
tests, the GUI tests and the interactive tests with all interpreters.
Set the default behavior of the unit test test runs to not modify the
configuration. This speeds up the quick runs of the tests, as it does not
disable the caching of the OutputConnectors. Thorough tests that run the
test suite with repetitions still run the subsequent runs with a modified
config.
Added helper function that creates a counting number string ("1st", "2nd",
"3rd", "4th" ...) from integer numbers.