Add utility for zMid

[cbegeman]

Compute zMid and zInterface from ssh and layerThickness and check whether it matches bottomDepth

Note: we may want to change this to computing from bottomDepth and layerThickness as is done in MPAS-Ocean but I found this way more useful for debugging Omega.

We may also want to return zInterface in the future.

Checklist

• API documentation in the Developer's Guide (api.md) has any new or modified class, method and/or functions listed
• Documentation has been built locally and changes look as expected
• Testing comment in the PR documents testing used to verify the changes

[cbegeman]

Testing

Testing has been done in the context of #435 on chrys, intel, openmpi with omega

[cbegeman]

@xylar Should be ready now if you want to rereview

[xylar]

Looks good. Just a few comments to consider.

[xylar]

@cbegeman, yes this looks good to me! Thanks for making those changes.