Integrate the regional application into the main branch

[xincjin-NOAA]

The regional application was originally developed as an independent package in [PR #116].

This branch extracts the regional application code from that implementation and integrates it into the main branch. This allows the regional application to benefit from ongoing model development and updates in the main OCELOT codebase.

Awaiting input data to verify correctness; however, this should not affect the global application workflow.

[xincjin-NOAA]

Summary from copilot

Main changes

• Regional mesh implementation

  • Adds gnn_model/create_mesh_graph_regional.py.
  • Builds flat or hierarchical 2D Cartesian meshes.
  • Projects latitude/longitude coordinates using an RRFS-style Lambert Conformal projection.
  • Adds regional observation-to-mesh and mesh-to-observation connectivity.
  • Normalizes regional mesh positions to [-1, 1] to avoid large-coordinate issues in reduced precision.

• Model integration

  • Adds mesh_geometry="global" | "regional" support to GNNLightning.
  • Adds regional domain bounds:
    • lon_min, lon_max
    • lat_min, lat_max
  • Adds configurable connectivity parameters:
  • mesh_cutoff_factor
  • mesh_num_neighbors
  • Preserves the existing global mesh path.

• Data-module changes

  • Dispatches mesh connectivity based on global versus regional geometry.
  • Makes missing training or validation bins fail explicitly with diagnostic errors.
  • Passes through the configured verbose value.

• Time-series processing

  • Replaces a sampled sortedness heuristic with an exact chunked check before using binary search on time arrays.
  • Adds fallback behavior when a valid time range contains no observations.

• Training CLI

  • Adds command-line options for regional mesh geometry and domain bounds.

• Testing and documentation

  • Adds gnn_model/tests/test_regional_mesh_smoke.py.
  • Adds gnn_model/regional.md with standalone, model-construction, and training smoke-test instructions.
  • Adds pyproj==3.6.1 to gnn_model/requirements.txt.