[Pafgbt] A little more PAF documentation

[Rick Fisher]

I've added a web page on using the software FX correlator on old ADLink 
style data.  It's current location is

http://www.cv.nrao.edu/~rfisher/ArrayFeed/software_correlator.html

Maybe the most useful point here is that it brings to life the FITS format 
for storing and archiving FX correlator data.  It should be fairly 
straightforward to adapt the correlator and FITS code to either "F" or "FX" 
output from the new BYU correlator.

The initial definition of the correlator FITS format can be found at

http://www.cv.nrao.edu/~rfisher/ArrayFeed/FXcorPAFfits.docx

I'd suggest that we use this format as the starting point for any further 
array data analysis since this is how GBT PAF data will be archived.

Rick