Updated documentation and P-harmonics picking in matplotlib plot

The documentation has been extended with a detailed description of the internal data format, a guide how to add an interface for a new tool and a brief description of the example data.

The P-harmonics lines in the Campbell diagram can now also be picked with mplcursors.

Enhancement of user settings

QSettings based user settings introduced, which can override the program defaults at start-up.

Initial release

Initial release of CampbellViewer GUI.