Merge pull request #1 from ellisowen/vela_data

Adds vela region fermi data to be called in gammapy.datasets.FermiVelaRe...
Adds new data files for fermi vela region, including background npred counts, counts, exposure and psf for the region
Updates README file for fermi vela region dataset
updates sh files outlining commands used to produce the data files
updates model file for fermi vela region