Implemented __call__

__call__ was implemented to determine the type of the object to read
(signal, signalgroup, timebase). The function determines the object type
and uses the appropriate getSignal, getSignalCalibrated, getSignalGroup
and getTimeBase to read the object. getSignalGroupCalibrated needs to be
implemented to read calibrated signalGroups.

Areabases and parameters sets should also be included.