Added signalGroup, fix getSignalGroupCalibrated

Similar to signal the class signalGroup is implemented. It provides min,
max, mean and median. mean has also been implemented for signal.
For signalGroups min, max, mean and median have the keyword axis which
works like the axis keyword in numpy.

axis = None --> global min, max, mean, median
axis = 0,1,... --> min, max, mean, median along this axis