Nodes of different colours represent the following:
Solid arrows point from a submodule to the (sub)module which it is
descended from. Dashed arrows point from a module or program unit to
modules which it uses.
READ INPUT FROM FILE AND DISTRIBUTE TO WORKERS.
STATE VARS ARE EXPECTED INCLUDE GHOST CELLS. NOTE ALSO
THAT RECORD-BASED INPUT IS USED SO NO FILES > 2GB DUE
TO GFORTRAN BUG WHICH DISALLOWS 8 BYTE INTEGER RECORD
LENGTHS.