Enh/pycortex features

Hi Chris,

Heres the PR for the new pycortex features in the NV codebase:
-symmetrical colormap ranges
-view all collection images in pycortex (which was a little bit tricky due to the undocumented architecture changes in pycortex)

I added a checksum of the file properties of a collection, so that if the collection contents change, the static pycortex output will be regenerated.

More to come!

Cheers,
Gabriel
can we put it into settings?