Psf analytical class

This implements a first version of `EnergyDependentMultiGaussPSF` and `EnergyDependentTableARF` classes. Test data and a corresponding `gp-irf-info` script is also contained.
This will not work for most users because they aren't at a location where `'gammapy/irf/tests/data/arf.fits'` exists.
You either have to move this file to `gammapy/datasets/data` and write a function to access it (see the examples in `gammapy/datasets/load.py`, or use `from astropy.utils.data import get_pkg_data_filename` directly here.

I think putting this in `datasets` is a bit nicer and later we can add e.g. a Fermi LAT ARF (see #71).