Spectral cubes

First update on spectral cube classes. Obviously still a lot to go - mainly tests.
Having these two headers here like this makes it seem to me that this is EIS crazyness. Ground based spectropolimetery is normally a 3D cube with one FITS header. I would suggest we make this API as close to `NDData` and `Map` as we can get away with because we will want to merge all these objects together before the magical 1.0