Added doc strings and dtype fail safes.

Doc string for __call__ added.
In case of getTimeBase, getSignalCalibrated and getSignalGroupCalibrated
a check is now performed if dtype is valid (libddww only allows float
and double for those routines). If an invalid dtype is provided then
numpy.float32 is used.
Merge pull request #22 from BSieglin/master

Added doc strings and dtype fail safes.