Update meshing submodule

[simbilod]

meshing code originally contained in the repo has been refactored to be pure GMSH and extended to 3D, see https://github.com/simbilod/meshwell

TODO

• [ ] rewrite examples to use the new API (very similar)
• [ ] make meshwell an (optional?) dependency
• [ ] remove legacy code

[HelgeGehring]

Sounds great!