Change Field vgrid metadata in MAPL3

[tclune]

Currently in MAPL3 the following information is stored about vertical structure of a field:

• num levels of grid (i.e., not of the field itself)
• vloc (staggering)

The following change should be implemented:

1. Field should store the number of levels it has. Most queries to a field want this not of the vertical grid and the existing design requires multiple accesses to info to calculate it.
2. Surface field should not have any entry for num levels. (Using 0 could be made to work, but ...)
3. "vloc" should be renamed "vstagger"
4. Parameters for staggering and the strings used for I/O should be colocated.