Added some tests for spectral cubes.
Added some tests for spectral cubes.

pyspeckit/pyspeckit,vlas-sokolov/pyspeckit,low-sky/pyspeckit,keflavich/pyspeckit,allisony/pyspeckit,low-sky/pyspeckit,e-koch/pyspeckit,e-koch/pyspeckit,keflavich/pyspeckit,vlas-sokolov/pyspeckit,jpinedaf/pyspeckit,allisony/pyspeckit,jpinedaf/pyspeckit,pyspeckit/pyspeckit