mpidummy: fix MPI_Reduce
Created by: germasch
The mpidummy implementation of MPI_Reduce didn't do the right thing, and doing the right thing actually makes it simpler.
During the maintenance period on Monday, 14 Oct 2024, all new projects will no longer create a container registry as part of the project area. Already existing projects will be unaffected at this time. Instead, users should utilize the official ORNL container registries at camden.ornl.gov (internal/moderate) and savannah.ornl.gov (external/low). Please see ORNL Today article for more info.
Created by: germasch
The mpidummy implementation of MPI_Reduce didn't do the right thing, and doing the right thing actually makes it simpler.