Add Gmsh Mesh Generation for subduction-3d Example

[draguve]

This pull request contains the code to generate the subduction-3d example using Gmsh instead of Cubit. The Gmsh-based workflow also incorporates topography for the region.

Topographic data is sourced from the GMTED2010 dataset. You can download the necessary files using the ./download_topography.sh script, which retrieves topography at a 30-arc-second spatial resolution.

To convert the topography into a usable NetCDF format, use the generate_localdem.py script. This script requires the GDAL package and is responsible for generating the NetCDF input consumed by generate_gmsh.py.

If a higher-resolution topography is desired, you can manually download the appropriate data and update the file paths in generate_localdem.py accordingly.

[baagaard-usgs]

Closes #731

[baagaard-usgs]

I am working on cleaning this up so that it can be merged.

• Rebase on current main
• Remove geometry at each step when no longer needed.
• Replace use of pyproj with spatialdata
• Refactor generate_gmsh.py Python script to break up create_geometry() into smaller chunks.
• Account for lateral variations in strike when adding contours up dip
• Replace use of topography only with topography/bathymetry from ETOPO2020.
• Update step 4 (getting zero pivot for Gmsh; remove splay fault from Dirichlet BC)
• Update step 6 (need to update z coordinates of GNSS stations to accont for topography)
• Update step 7
• Update step 8
• Update Cubit mesh (Python script; update names match Gmsh)
• Clean up Python scripts and cfg files. Remove Cubit journal files.
• Update examples documentation (Gmsh, Cubit Python script).
• Expand coverage of slow slip spatial database (Cubit mesh).