Probe

[mohalinasre]

Implemented the probe post-process type, which until now was scaffolded but stubbed (errorMsg("Probe is not implemented yet")). A probe samples a nodal field at a fixed point in space and writes a time history, independent of the results.e field dump, so a single location can be logged at every timestep without forcing full-field output at that cadence.

Under domain decomposition the probe point lives in exactly one element on one rank. All ranks search locally; the ranks then agree on the single owning rank by a global argmin (MPI_MINLOC) of the isInElement distance among ranks that actually contain the point. Only the owner samples, and its value is reduced to master, which writes the file. A point owned by no rank at all (outside the mesh) raises a collective error decided globally, so a non-owning rank is never mistaken for a fatal "not found."

Output columns are named by field: a scalar writes the field name (pressure); a spatial vector writes field_x, field_y, field_z; anything else falls back to numbered components.

How to use it (input.i):
The probe is declared as an entry in the post_process array under solver → output_control, alongside output_fields:

output_control:
            file_path: results.e
            output_frequency:
                option: time_interval
                time_interval: 0.05
            output_fields: [velocity, pressure, displacement_mesh]
            post_process:
            - name: pressure_probe
              location: [fluid]
              frequency: 1
              write_to_file: true
              type: probe
              options:
                  probe_location: [0.28, 0.024]
                  field: pressure

Validation
Serial-validated on the rigid dam break (pressure) and the flexible dam break (displacement, fixed and moving mesh), and parallel-validated on the rigid dam break.
Not tested in parallel on the flexible case: that case independently segfaults in parallel at the non-conformal interface transfer (XFER::LinInterp::fine_search), with or without the probe present. This is a pre-existing interface-transfer issue unrelated to this change.

[mohalinasre]

The PR has two commits, the first is the probe implementation, the second is a follow-up fix adding correct parallel ownership (single-rank sampling via global argmin) after I found it crashed in decomposed runs; they review as one combined diff.