Vela corrections

Makes the corrections to the fermi analysis we discussed... although npred counts now goes down to 145 which seems low?
Did you choose `emin=50` here on purpose?
This gives a 28 MB event list with tons of events below 10 GeV.

I'd prefer you use `emin=10000` and then have an event list that matches in the energy range with the counts cube. (the number of events in the counts cube will still be smaller because the circular ROI from `gtselect` is larger than the rectangular region from the counts cube)