Write grid variables only once.

[dabail10]

One thing that has annoyed me for years is the missing values in land (from land block elimination) in the grid variables. Several users have requested that we fill in the land blocks with valid values. Mostly for analysis after the fact. One way to do this is how they do it in CESM-POP. There is a single file with the string "once" in it, where the time constant variables are written only once. This would be written in ice_grid.F90 when the global arrays are read in. This is still CF-compliant in the sense that this file would always accompany the history files. It also would save some space not writing all the grid information to every history file.

[anton-seaice]

I "think" we currently only have the missing values during the early initialization (i.e. during init_grid2) so the easiest thing would be to create and write the one-off / fixed frequency grid output file during that subroutine.

If we want to keep (in memory) the coordinates for eliminated land blocks to include in the history output, then the change gets a bit more complicated!