add mapper functionality to `sim_transport.h`

[jmlapre]

sst-elements/src/sst/elements/iris/sumi/sim_transport.h

Lines 124 to 126 in d6244da

	// FIXME
	//return rank_mapper_->rankToNode(rank);
	return SST::Hg::NodeId(rank);

Borrow and modify a mapper from nearby code.