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
mutli gauss class, effective area class and irf info script
Added test data for ARF and PSF
ARF and PSF tests
Changed pyfact to use new arf class