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. 
Sorry no I can't think of a way to refactor this. To plot an slice of the data cube, the test has ```image = fits.getdata(self._data_cube)``` and ```w = WCS(self._data_cube)``` and it uses both ```w``` and ```image``` so I decided returning the fits file itself was a good idea.