Adds main npred cube script to spectral_cube core.py
Adds unit test scripts for spectral_cube npred methods
Adds main rst script for a tutorial for the npred cube methods
Adds plotting scripts for the npred method tutorial
Adds Fermi Science tools reference to references.txt file
Merge branch 'npred_cube2' into image_from_catalog

Conflicts:
	docs/tutorials/index.rst
	gammapy/spectral_cube/core.py
	gammapy/spectral_cube/tests/test_core.py