Gridcell-to-gridcell comm V0: move grid check out of elm_drv for performance improvement

[glemieux]

Currently the seed dispersal code recomputes the nearest neighbor locations every timestep in clm_drv which is inefficient. Setup a mechanism to record this as global or gridcell type vector to be calculated during initialization.

[glemieux]

Q: When is ldecomp initialized? Neighbor algorithm needs to be defined after this.

• decomMod.F90 defines the decomp_type
• decompInit_lnd procedure allocates the ixy/jxy variables
• Initialized via decompInit_lnd @ 177 in initialize1

Q: When is the gridcell type initialized? If we want to calculate neighbors once and store in gridcell, needs to come after gridcell allocation, which I think comes after decomp.

• note: ctsm does not have elm's GridcellDataType, but both share GridcellType which defines "physical" properties
• Initialized via grc_pp%Init @ 320 in initialize1
• this initialization simply nans values. The actual initialization is via initGridcells call in iniGridCellsMod @ 351 later in initialize1

[glemieux]

Closing this with 31fb53b