Xarray-like coords attribute

[DanRyanIrish]

We should have an attribute on Cube and/or CubeSequence that does something similar to the Xarray coords attribute, i.e. holds arrays (or functions that return arrays) of the coordinate values and the axis of the Cube/CubeSequence to which they apply. For example, Cube.coords["wavelength"] could give the wavelength quantity of the spectral axis. And Cube.coords["time"] could give the time of each exposure along that axis.