CF coord systems

[bblay]

Fixes #45

[bblay]

Not sure about introducing a default ellipsoid. We might not be saving the same thing as we loaded...

[bblay]

One outstanding doctest failure:
From surface_altitude: Cell(point=399.625, bound=(-14.0, 813.25)) m
to surface_altitude: Cell(point=399.625, bound=(-14.000001, 813.25)) m
under investigation.

Looks like a str() vs repr() but not sure how that could be happening yet.

[rhattersley]

This is really a comment about the GeogCS class/constructor. Can we ditch the units keyword - CF mandates the radius and major/minor axes are expressed in metres. And the GeogCS constructor should probably cast to float to be on the safe side. That would give:

lat_lon_coord_system = icoord_systems.GeogCS(6371229)