Should MAPL_Comms routines support 3d and 4d?

[mathomp4]

Recently, in helping @briardew try various things to get his code working on SLES15, he pointed me to the CO code:

https://github.com/GEOS-ESM/GOCART/blob/76a694c11f0e69ace310139770094981732d1476/ESMF/GOCART_GridComp/CO_GridComp/CO_GridCompMod.F90#L777

which has that sort of code that was in PChem: A parallel read of a file that we converted to read-on-root-and-broadcast.

Well, I did the same sort of thing and one thing I did to be "smart" was convert:

  length = SIZE(gcCO%sdat)
  CALL MPI_Bcast(gcCO%sdat, length, MPI_REAL, 0, comm, status)
  VERIFY_(status)

to:

  length = SIZE(gcCO%sdat)
  CALL MAPL_CommsBcast(vm, gcCO%sdat, length, 0, comm, status)
  VERIFY_(status)

But that failed. Why? Well, gcCO%sdat is 4-d:

        REAL, POINTER :: sdat(:,:,:,:)

and a couple other of them are 3-d. And MAPL_CommsBcast is overloaded only to 2d:

MAPL/base/MAPL_Comms.F90

Lines 91 to 114 in c65548b

	interface MAPL_CommsBcast
	module procedure MAPL_CommsBcast_STRING_0
	module procedure MAPL_CommsBcast_L4_0
	module procedure MAPL_CommsBcast_I4_0
	module procedure MAPL_CommsBcast_R4_0
	module procedure MAPL_CommsBcast_R8_0
	module procedure MAPL_CommsBcast_I4_1
	module procedure MAPL_CommsBcast_R4_1
	module procedure MAPL_CommsBcast_R8_1
	module procedure MAPL_CommsBcast_I4_2
	module procedure MAPL_CommsBcast_R4_2
	module procedure MAPL_CommsBcast_R8_2
	module procedure MAPL_CommsBcastVm_STRING_0
	module procedure MAPL_CommsBcastVm_L4_0
	module procedure MAPL_CommsBcastVm_I4_0
	module procedure MAPL_CommsBcastVm_R4_0
	module procedure MAPL_CommsBcastVm_R8_0
	module procedure MAPL_CommsBcastVm_I4_1
	module procedure MAPL_CommsBcastVm_R4_1
	module procedure MAPL_CommsBcastVm_R8_1
	module procedure MAPL_CommsBcastVm_I4_2
	module procedure MAPL_CommsBcastVm_R4_2
	module procedure MAPL_CommsBcastVm_R8_2
	end interface

So I suppose my questions are:

1. Is there a reason we only overload to 2d?
2. If not, is there a reason we haven't overloaded higher?