For issue #48

This is for issue #48. Currently this downloads the larger cube 'L1448_13CO.fits' because that's the file used in the image tests in the main repository. 
No problem! It might be worthwhile just having a read over the astropy.io.fits docs at http://docs.astropy.org/en/stable/io/fits/index.html to understand a bit better how all the data/header stuff can be accessed.