Fixes array broadcasting in spectral cube and adds unit test

Fixed doc links
Add crab_mwl_sed tutorial with plot
Merge pull request #141 from ellisowen/spec_cube_bug_fix

Spec cube bug fix
Remove xfail marker from tests that pass
Sqrt stretch colormap_hess
Script for model image computation
toy galaxy modeling
Update
Implemented distributions as astropy models
Changed all models to astropy models added testing and docs