Psrfits2fil

Hi Scott,

This branch adds the psrfits2fil.py script which converts psrfits files into filterbank files. It was created as part of Patrick's inject branch, but I've cherry-picked the appropriate commits into a separate branch since we've had some requests for it. There are quite a few commits, but the net effect is the addition of psrfits2fil.py and filterbank.py.

Paul