Adds method to gammapy.image.utils for fermi psf convolution

Adds method for fermi energy-dependent psf convolution in gammapy.image.
This function should accept an `EnergyDependentTablePSF` as input or be a method on that class.

The PSF you use here will only be correct for a given source class and change e.g. when PASS 8 comes out.
Plus we want to use the same PSF class e.g. for HESS and CTA and will need to use those as well.