PTB-Replacement for numel() on older Matlab versions that don’t support it.
PsychNumel is a drop-in replacement for the numel() function of recent
versions of Matlab. If called on modern Matlabs, it will just call
numel(). On older Matlabs it will emulate the behaviour of numel().
n = PsychNumel(x); will return the total number of elements contained in
scalar, vector or matrix x, i.e. n == prod(size(x));