Xcorr

Hi Marten

The x-corr code is in place (though could use some tidying and more docstrings). You'll recognize most of it from folding.py, though now we loop over the filehandles to setup/configure all the channelization stuff.

There are a few places we need to think about the physics, but most things work. 

I'm pushing this now but we can go over it tomorrow morning.

Cheers,
-Aaron