Adds to gammapy.datasets scripts to load fermi extended source catalog images
Adds fermi extended sources test case
Adds general PSF convolution method to gammapy.image.utils
Adds unit test for new PSF convolution method in image utils
Merge branch 'psf_convolve' of https://github.com/ellisowen/gammapy into catalog_table
Merge branch 'extended_sources' of https://github.com/ellisowen/gammapy into catalog_table