Add mesh-files-only internal spherical shell generation

[gthyagi]

Summary

• extract the existing atomic Gmsh-to-PETSc-HDF5 conversion into _gmsh_to_h5();
• add SphericalShellInternalBoundary(write_mesh_files_only=True);
• return the generated .msh.h5 path without constructing an in-memory UW3 Mesh;
• preserve the existing full-Mesh behavior by default;
• add focused regression coverage for bypassing Mesh construction and reloading the generated HDF5 with Lower, Internal, and Upper labels.

Motivation

SphericalShellInternalBoundary() currently writes the reusable .msh.h5 file during Gmsh import, then reloads that file into a full UW3 mesh and materializes Inner and Outer cell-region labels. The region creation loops over every cell in Python.

That work is required when the caller needs the returned in-memory mesh, but it is unnecessary for workflows that only generate reusable mesh files for later MPI jobs. The custom region labels are created after the HDF5 write and therefore do not alter the already-written reusable file.

This became significant for the Zhong et al. spherical-shell benchmark on Gadi. Each cellSize=1/64 mesh contains about 4.2 million cells. The approximately 710 MB HDF5 files appeared several minutes before the serial jobs exited; one measured job completed in 15:14 using 8.35 GB while remaining CPU-bound in post-conversion mesh setup.

API

h5_path=uw.meshing.SphericalShellInternalBoundary(
radiusOuter=1.0,
radiusInternal=0.775,
radiusInner=0.55,
cellSize=1.0/64.0,
filename="shell.msh",
write_mesh_files_only=True,
)

With write_mesh_files_only=True, the generator:

1. writes the Gmsh .msh file;
2. atomically converts it to PETSc .msh.h5;
3. preserves the imported physical boundary labels;
4. returns the HDF5 path as a string;
5. skips HDF5 reload, coordinate-system initialization, and cell-region classification.

The default remains write_mesh_files_only=False, so existing callers continue to receive a fully initialized UW3 Mesh with no behavior change.

Validation

Validated on a fresh branch from upstream/development at 59982834:

• ./uw build completed successfully;
• file-generation regression passed;
• unchanged full-constructor boundary-area regression passed;
• focused result: 2 passed;
• Python syntax and whitespace checks passed.

The same feature diff also passed the complete level-1 suite before being squashed for this PR: 1493 passed, 33 skipped, 2 xfailed.