Add a 'quicklook' viewer for slices

[keflavich]

Add an aplpy-based quicklook tool. E.g.:

cube = SpectralCube.read('cube.fits')
cube[50,:,:].quicklook()
cube[25:50,:,:].sum(axis=0).quicklook()
cube.spectral_slab(0*u.km/u.s, 10*u.km/u.s).quicklook()

[keflavich]

I'm not adding tests for this since it is matplotlib-related and fairly basic. If anyone has a good idea for a test that should be written for this, please add it!

[astrofrog]

I wonder whether it would make more sense to use WCSAxes here? What do you think?

[keflavich]

Sure - I'm just not as familiar with it yet. Can easily be swapped in.

[astrofrog]

@keflavich - ok, I'll do it at some point soon.