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 is the only method (apart from the plot methods) that's not tested.
Can you add a test that writes this to a temp file and reads it back?
(there are examples how to get the temp filename in gammapy ... search form `tempfile`)

If you have the energy you could add an assert for something, but in my experience such methods usually break because of API changes elsewhere (e.g. in Astropy), so simply executing it is useful even without asserts.