`BottomDepth` should not be read from the horizontal mesh

[xylar]

The BottomDepth field is not part of a standard MPAS horizontal mesh, so it should not be required to be in the horizontal mesh file. Like other vertical mesh fields, it should instead be in the vertical mesh file.

[xylar]

It also should be allowed to be BottomDepth, not bottomDepth.

[cbegeman]

I think #328 at least partially but perhaps fully addresses the issue. @brian-oneill can you confirm/elaborate?

[brian-oneill]

Correct, BottomDepth was added to the VertCoord in #275, but remained in the HorzMesh and was still being read in there. The remaining member arrays and the mesh file read are removed from HorzMesh in #328.

[xylar]

Okay, wonderful. I'll add that note in #328.