Mark IndexMaps on communicators with size 1 as non-overlapped

[garth-wells]

For Functions on an MPI communicator of size 1, this change reduced the number of communicators that are created.

[chrisrichardson]

Can we get rid of _overlapping and just store the comm size (or ask MPI for it)? I don't think we ever use an IndexMap with no overlap, except when in serial. At least, the constructor IndexMap(comm, local_size) doesn't seem to be used internally.

[jorgensd]

Can we get rid of _overlapping and just store the comm size (or ask MPI for it)? I don't think we ever use an IndexMap with no overlap, except when in serial. At least, the constructor IndexMap(comm, local_size) doesn't seem to be used internally.

I use that indexmap ( IndexMap(comm, local_size)) in some of my codes. I guess I can adapt it to sending in no ghosts.

[garth-wells]

I use that indexmap ( IndexMap(comm, local_size)) in some of my codes. I guess I can adapt it to sending in no ghosts.

It’s used in a few tests too, so I think we should keep the constructor.