new function to add phase jitter to a dataset

Here is a new function that intrduces pahse jitter to a raw dataset. I have found it useful to creating null datasets for statistical comparasons or for testing effects of phase-jitter. It can introduce  uniform or von-mises-distrubuted phase jitter, paramaters of which can be specifed either across the entire spectrum or variable across freqencies. 

If you have any questions or concerns please let me know.

Thanks

Mark Drakesmith