Spectral cubes

First update on spectral cube classes. Obviously still a lot to go - mainly tests.
I'm not sure what would be the best way... and I was going to write all about it in the astropy mailing list... - well... I have to do it sooner than later.
In any case, just to explain it now to @Cadair, the thing is that EIS, and many other spectrograms have a set of 3D cubes (windows) which will have their independent header - though it should not change much to the global one.