Enable specification of ModelLevelsVerticalGrid in ComponentSpecParser

[tclune]

The challenge here is that a ModelLevelsVerticalGrid needs a pointer to the StateRegistry that owns the underlying variables that define the vertical coordinates (e.g., PLE).

One possibility is that the registry is passed into the parser, but that feels rather clunky. Last resort.

Another possibility is to do a 2-step initialization. During the parsing we create the object but leave the registry unset. We then have logic later (somewhere in OuterMetaComponent) that sees the vertical grid is ModelLevels and calls set_registry(). Also very clunky and a last resort.

Yet another possibility is to introduce a VerticalGridSpec and a VerticalGridManager analogous to the existing (horizontal) GridManager. The spec can then hold the things that are specified in yaml, but the actual creation of the VerticalGrid in the manager could add in the registry at that time. Lots of handwaving here.