Read the replica index on rank 0 in DIMER, grid output and OUTPUT_ANALYSIS_DATA_TO_COLVAR (#1443)

[aalhossary]

Description

The v2.9 part of #1443. multi_sim_comm is valid only on rank 0 of each replica, but three actions read it on every rank. With more than one MPI rank per replica (domain decomposition) they get the wrong replica index on the other ranks:

action	symptom
DIMER	aborts: "Mismatch between provided sigmas and number of replicas"
DUMPGRID, DUMPCUBE, GRID_TO_XYZ (GridPrintingBase)	replica 1 also writes its grid although REPLICA=0 (histo.1)
OUTPUT_ANALYSIS_DATA_TO_COLVAR	replica 1 also writes its data although REPLICA=0 (frames.1)

The plan on #1443 was to guard such actions with an explicit error. Reading the index on rank 0 and broadcasting it over comm is the same few lines and makes them work instead, so this fixes them. One commit per action.

I checked every other use of multi_sim_comm on v2.9: they either use the same rank-0 pattern already or run inside rank-0 blocks. LOGMFD is tracked separately in #1480. On master these actions use the helpers from #1481 (DIMER) or no longer exist (the other two).

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.

Three new regtests, each with 2 replicas × 2 MPI ranks under driver --debug-dd yes:

• basic/rt-dimer-multi-dd (same as on master): forces match the 1-rank-per-replica run exactly. Without the fix it aborts.
• basic/rt-dumpgrid-multi-dd and basic/rt-output-colvar-multi-dd: output matches the 1-rank-per-replica run, and the list of files written is checked. Without the fix an extra .1 file appears and the test fails.

Locally the full suite gives 614 performed and 2 errors, both Fortran tests that are unrelated to this change: rt-make-fortran (my gfortran rejects the argument type mismatches in the legacy interface) and rt-make-fortran08 (as on master).