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.
Quantity input validation is starting to get tedious.

Should we bundle the [validator.py](https://github.com/zblz/gammafit/blob/master/gammafit/extern/validator.py) file that @astrofrog wrote in `gammapy` ... or Tom, do you think it should go directly in Astropy?