PlotPanels and -Windows for plotting iterable sequences like tuples or lists
An improved implementation for the spectrum's group delay calculation
Improve the GUI's threading issues
 - Make sure that the wx MainLoop always runs in the same thread
 - Run more functions in the MainLoop
 - Use Events instead of Locks in some places
 - Replace wx.YieldIfNecessary with time.sleep in the window's Join method

API-Changes: The functions sumpf.gui.is_mainloop_running and join_mainloop
are no longer available. The start_mainloop function is replaced by the
bump_mainloop function

The threading issues have been significantly improved by this patch. The
unit tests for the gui are passed for more than half the times that they
are run (Before, they have never been passed). Nevertheless, there are still
some issues in the GUI.