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
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 references.rst file in docs to include FSSC Fermi Tools reference