computePSTH completed

Format:
 	PSTH:	{# atten vals, # stim groups}, where each element is a matrix
of size [# sweeps (reps), # bins]
 bins:	[1, length(psthwin(1):binsize:psthwin(2))] vector of time bins
for PSTH.
 spiketimes:{# attenvals, # stim groups}  cell matrix, each element is
a cell vector, {# sweeps}, each containing a vector of spiketimes (in
milliseconds) for that sweep;