Vela corrections

Makes the corrections to the fermi analysis we discussed... although npred counts now goes down to 145 which seems low?
Here you only select events within 1 deg of Vela.
But your cube spans a 5 deg x 5 deg region.
This means your actual counts will not be comparable to the model counts in the cube.

For a binned analysis you have to do what is described here:
http://fermi.gsfc.nasa.gov/ssc/data/analysis/scitools/binned_likelihood_tutorial.html#create3DMap

I.e. select events in a circular region that completely covers the cube ... in this case `sqrt(2) * 2.5 = 3.53` so running `gtselect` with `rad=4` should be OK.