Specify short name of vertical grid coordinate in yaml file, instead of hardwiring them in parse_geometry_spec

[pchakraborty]

These cannot be hardwired here. At least not in the long term. How about requiring them in the yaml file when you try to specify the vertical grid that way:

vertical_grid:
    coordinate:
        dimension: pressure
        stagger: edge
        field: PLE
    coordinate:
        dimension: height
        stagger: center
        field: ZL
...

Originally posted by @tclune in #3182 (comment)