Implemented distributions as astropy models

Conflicts:
	gammapy/astro/population/spatial.py
Changed all models to astropy models added testing and docs
Changed code to use quantities
Merge pull request #157 from adonath/astropy_population_models

Galaxy modelling code revision
Adds new methods to GammaSpectralCube and adds npred cube method to gammapy spectral_cube core.py
Adds new unit tests to spectral_cube for the new methods added to spectral_cube core.py
Updates datasets load script and unit test to account for change in filename of remote data cubes, and additional Vela remote data files
Adds normalize kernel option to irf psf_table EnergyDependentTablePSF and unit test
Adds predicted counts cube tutorial, including plotting scripts and updates doc index.rst file to accomodate new addition
Updates tutorial index rst page for new npred tutorials
Updates references.rst file in docs to include FSSC Fermi Tools reference