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.
Put this code that computes the containment radius array into a separate function.

It can be a private function for now (i.e. start with underscore) so you don't have to think about API and write a docstring, but it's always good to split computation from plotting so that it's more re-usable.