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