Document comm and multi_sim_comm (#1443, part A)

[aalhossary]

Description

Part A of #1443: document the replica communicators so their contract is visible to both action authors and MD-code authors. Documentation only; no code changes.

• Action::comm: the ranks of this replica; valid on every rank.
• Action::multi_sim_comm: one rank per replica, valid only on rank 0 of comm. Elsewhere it is MPI_COMM_SELF, which reports one replica and reaches no other replica. Includes the read-on-rank-0-and-broadcast pattern.
• PlumedMain::comm / multi_sim_comm: a short note plus a pointer to the Action comments.
• GREX::intracomm / intercomm: what each holds, and that setMPIIntercomm also sets PlumedMain::multi_sim_comm and is passed only on rank 0.
• developer-doc/mdTemplate.md: new section Dealing with multiple replicas next to Dealing with parallelism. It shows which calls an MD code makes on which processes (taken from driver) and what each process then sees inside PLUMED. It is linked from Action::multi_sim_comm with \ref mpireplicas.

When this is merged into master, the comment can also point to getNumberOfReplicas() / getReplicaIndex() (#1481).

Target release

I would like my code to appear in release v2.9

Type of contribution

• changes to code or doc authored by PLUMED developers, or additions of code in the core or within the default modules
• changes to a module not authored by you
• new module contribution or edit of a module authored by you

Copyright

• I agree to transfer the copyright of the code I have written to the PLUMED developers or to the author of the code I am modifying.

Tests

• I added a new regtest or modified an existing regtest to validate my changes.
• I verified that all regtests are passed successfully on GitHub Actions.