Added average subtraction to getPSD function within freqProc.py.

Added fs if statement to piezofunctions.py and totalDataProcessing.py to
allow for a choice of sampling frequency to be passed from the
programLaunch.py module to the butterworth filter within
piezofunctions.py.

Removed averaging in the piezo and accelerometer getAutoCorrelation
functions in totalDataProcessing to give the highest resolution autocorr
plot.