Simudo

Added
- `fem.debug_probe` and `MeshUtil.get_debug_probe` to probe physical
quantities albeit inefficiently.
- Add jupyter notebook with examples.
- Update user docs with instructions for Docker on Windows and
Podman on GNU+Linux.

Changed
- Use absolute imports in `fourlayer_example` to help out users who
just want to modify the geometry without messing with the material
parameters much.

Added
- Physics: Thermionic heterojunction boundary condition.
- FEM: Utility method `MeshUtil.region_oriented_dS()` for getting the
same-side and opposite-side dS facet measures from a FacetRegion.
- Mesh: Null cell and facet regions.
- Added changelog file.

Changed
- Stepper: If the solver raises a RuntimeError, treat it as a failure
rather than letting the exception bubble up.
- Stepper: Reduce step size more dramatically on failure.