[Mitgcm-support] flat packages

[mitgcm-support at dev.mitgcm.org]

Incidentally, if anyone else ever packages part of the model, the packages
need not be flat. e.g. (hypothetically)

 % ls pkg
  obcs
  obcs/radbcs
  exch
  exch/lowlevel
  exch/jam

 % genmake -disable=obcs/radbcs,exch/jam

I'm not suggesting we add much hierarchy to the pkg tree but a
particular package might be better described in sub-units...

Other usage info:
The subdirectories are essentially treated as separate packages
but they inherit the disabled option from their parent:
   -disable=obcs   disables obcs and obcs/radbcs
The enable option is not inherited since packages are enabled by
default so that:
   -disable=obcs -enable=obcs is short hand for
   -disable=obcs/radbcs,obcs/observ_data,obcs/jmc_special_rad