Correct signal file sample range

[bemoody]

In some cases, the stated sample range for a signal (scale_lower/scale_upper) is flat-out wrong. ECG signals in particular are often wrong.

WaveSampleHandler should calculate and report an accurate sample range (adcres/adczero) for each segment. This is required in order to correctly convert the record to other formats (e.g., using wfdb2mat.)