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 derived vertical coordinate is *ocean_sigma_z_coordinate*. However, the CF community seem to have failed to register this as a valid *standard_name* :poop: 

As a follow-on task, I'll communicate this discrepancy and lobby for its definition and inclusion in the official CF Standard Name Table.