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. 
The data cube? Yes, to plot a slice of the cube, we need to do 
```image = fits.getdata(self._data_cube)``` and ```w = WCS(self._data_cube)``` so I decided to return the fits file as it is. Do you have suggestions on how to do this differently?