Ocean sigma over z

This PR provides support for the CF **ocean sigma over z** dimensionless vertical coordinate.

As per the CF Metadata Conventions the *standard_name* of the dimensonless vertical coordinate is **ocean_sigma_z_coordinate**. However, the CF community seem to have failed to register this as a valid *standard_name* :cry: 

As a follow-on task, I'll communicate this discrepancy and lobby for its definition and inclusion in the official CF Standard Name Table.

I'm also not 100% sure whether the derived coordinate *standard_name* of **sea_surface_height_above_reference_ellipsoid** is appropriate. Confirmation welcomed.