Only resize vector on root rank we're reducing to.

As Steve Nowlen (drnuke@lanl.gov) pointed out, we can
avoid the vector resize on all ranks except the root we're
reducing to.
The documentation at mpi-forum moved from http://www.mpi-forum.org/docs/mpi-11-html to http://www.mpi-forum.org/docs/mpi-1.1/mpi-11-html. Changed the documentation to reflect that.
In short term, we might want to move to MPI 2.x documentation.
Merge branch 'map-c-update' into cartesian
Small typo, we had double slashes in external URL.
Did not hurt, but it nicer this way.
Merge branch 'map-c-update' into cartesian
added mapping documentation for MPI_Cart_create, MPI_Dims_create MPI_Cartdim_get.
Added documentation for C API mapping
Added down cast functions for communicator to cartesian communicator conversion.