allReduce fails with GeomVector

[brbass]

Calling FieldList<Dimension, Vector>::sumElements causes an error when Spheral is compiled in MPI mode. Specifically,

Fatal error in MPI_Allreduce: Invalid MPI_Op, error stack:
MPI_Allreduce(937)......: MPI_Allreduce(sbuf=..., rbuf=..., count=1, dtype=USER<contig>, MPI_SUM, MPI_COMM_WORLD) failed
MPIR_SUM_check_dtype(97): MPI_Op MPI_SUM operation not defined for this datatype

To reproduce, call sumElements on a FieldList<Dimension, Vector>. The same issue could occur for SymTensor/Tensor as well. The code controlling the MPI behavior for Spheral types is here:

spheral/src/Utilities/RegisterMPIDataTypes.cc

Lines 23 to 28 in 7abbe6c

	MPI_Type_contiguous(DataTypeTraits<Dim<1>::Vector>::numElements(Dim<1>::Vector::zero), MPI_DOUBLE, &MPI_Vector1d);
	MPI_Type_contiguous(DataTypeTraits<Dim<2>::Vector>::numElements(Dim<2>::Vector::zero), MPI_DOUBLE, &MPI_Vector2d);
	MPI_Type_contiguous(DataTypeTraits<Dim<3>::Vector>::numElements(Dim<3>::Vector::zero), MPI_DOUBLE, &MPI_Vector3d);
	MPI_Type_commit(&MPI_Vector1d);
	MPI_Type_commit(&MPI_Vector2d);
	MPI_Type_commit(&MPI_Vector3d);