Implement local VTU export for load balancing

[Geethapranay1]

Fix #189

This PR implements a native, local VTU export functionality specifically tailored for dynamic load balancing, resolving the previously reported export data loss and duplicate points issues.

When load balancing and export:vtu are enabled concurrently:

• we automatically intercept the export:vtu tag in the preCICE configuration and parse its parameters (directory, frequency).
• MicroManagerCoupling suppresses preCICE's native global export by injecting a modified, stripped XML locally.
• micro-manager now handles it natively: every MPI rank iterates solely over its currently owned/active subset of coords_local and writes proper .vtu (VTK_VERTEX) files containing exactly its local scalar and vector datasets.
• rank 0 orchestrates gathering the MPI metadata and produces the coordinating parallel .pvtu file.

Checklist:

• I made sure that the CI passed before I ask for a review.
• I added a summary of the changes (compared to the last release) in the CHANGELOG.md.
• If necessary, I made changes to the documentation and/or added new content.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.

Testing

Successfully tested VTU generation offline. Example output format validation when run across distributed domains:

Click to view generated VTU Structure Proof

Local VTU format (e.g. Micro-Manager_Macro-Mesh_1_rank0.vtu):

<?xml version='1.0' encoding='utf-8'?>
<VTKFile type="UnstructuredGrid" version="0.1" byte_order="LittleEndian">
  <UnstructuredGrid>
    <Piece NumberOfPoints="3" NumberOfCells="3">
      <Points>
        <DataArray type="Float64" NumberOfComponents="3" format="ascii">0.0 0.0 0.0 1.0 0.0 0.0 0.0 1.0 0.0</DataArray>
      </Points>
      <Cells>
        <DataArray type="Int32" Name="connectivity" format="ascii">0 1 2</DataArray>
        <DataArray type="Int32" Name="offsets" format="ascii">1 2 3</DataArray>
        <DataArray type="UInt8" Name="types" format="ascii">1 1 1</DataArray>
      </Cells>
      <PointData>
        <DataArray type="Float64" Name="Temperature" NumberOfComponents="1" format="ascii">100.5 110.2 95.8</DataArray>
        <DataArray type="Float64" Name="Velocity" NumberOfComponents="3" format="ascii">1.0 0.5 0.0 1.2 0.6 0.1 0.9 0.4 0.0</DataArray>
      </PointData>
    </Piece>
  </UnstructuredGrid>
</VTKFile>

[Geethapranay1]

hi @IshaanDesai
I have pushed a new commit to address the remaining edge cases, stabilize the load-balanced VTU exporter.

here are the main improvements:

• replaced fragile ElementTree parsing with targeted text parsing to safely handle preCICE <export:vtu /> tags without XML namespace issues.
• the custom VTU writer now properly supports empty ranks, writing the necessary files so the .pvtu output remains valid and unbroken.
• enforced a globally consistent point-data schema across all ranks (using allgather) to ensure seamless loading in ParaView.
• fixed the vertex ID mapping to use the actual preCICE vertex IDs (_mesh_vertex_ids) rather than raw coordinate indices.
• added missing unit tests covering empty-rank VTU generation, 2D-to-3D vector padding, and .pvtu file creation.

Note: full feature parity for every-iteration="true" is not yet implemented in the custom exporter (it currently exports completed time windows only), but it now safely logs a warning if configured.