todo: note that this wont work if the gains are different on channels on the same ground. returns the average of a number of raw signals; the purpose of the returned value is to-renormalize to a common ground, in case the ground wire was noisy. input params: datapath channels: array of channels to be used for averaging. this should not include the ground channel itself. fromInd/tillInd: which blocks to read, in units of 512 datasamples (dependent on sampling rate) returns: dataAll is the normalized mean data is the data itself urut/april06