Added the ability to normalize the correlation.

The corr3_parallel function has a new prototype:

void corr3_parallel(double *f1, double *f2, int n_f1, int n_f2, int tau1, int
tau2, int unbiased, double *xcorr);

The int unbiased term is a boolean which selects whether to use "unbiased"
normalization. This normalization simply divides the resultant sum by the
length of the overlap between the two arrays. unbiased = 1 gives this behavior.
Merge branch 'master' of github.com:smattacus/data-analysis