Added area to signal and signalGroup class.

signal and signalGroup now contained area.
If the singal / signal group has no area base it is None.
If it has one area base it is the area base.
If it has more than one it is a dictionary containing the area bases.
updated documentation to reflect new changes
Added header to signal and signalGroup.

The corresponding object header is now added to the signal and
signalGroup class.

The decorator description returns the information text of the signal /
signalGroup.
Merge documentation and header changes.
added more sophisticated 2d example, some more fixes to docs
typos fixed
Fixed timebase in signalInfo and signal operators.

getSignalInfo now returns the timebase name if a timebase is specified.

signal operators now work again, header is included.
Merged typo
Fixed error in dd.shotfile.open

For some discharges it was not possible to open specific shotfiles if
other discharges of the same diagnostic were opened before.

This was the result of an error in ddlastshotnr_ which was used to check
if the shotfile for the desired discharge is available. This check has
been removed. If the shotfile is not available an exception is thrown.