Add unstructured finite volume spatial method

[aabills]

Summary

• Add FiniteVolumeUnstructured spatial method, discretisation hooks (div_D_grad, internal BC dispatch), and unstructured ProcessedVariable* classes.
• Depends on Generalise VectorField to N components #5686 (VectorField) and Add unstructured mesh infrastructure #5687 (Meshing); both are merged into this branch.

Stack

1. Generalise VectorField to N components #5686 — VectorField N-comp
2. Add unstructured mesh infrastructure #5687 — Meshing
3. This PR (Add unstructured finite volume spatial method #5688) — Spatial method + ProcessedVariable
4. Add VTK plotting for unstructured meshes #5689 — VTK plotting
5. Add unstructured 2D/3D DFN battery models #5690 — Unstructured DFN models

Review tip: the spatial-only commit is feat: add unstructured finite volume spatial method; earlier commits are the two dependencies.

Test plan

• test_finite_volume_unstructured.py + mesh + tensor-field tests (158 passed locally)
• CI unit suite

Also in this stack: #5691 deprecates pybamm.Magnitude.

[codecov]

Codecov Report

❌ Patch coverage is 77.07663% with 356 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.24%. Comparing base (1e9fbce) to head (7dc6f0d).

Files with missing lines	Patch %	Lines
...es/pybamm/src/pybamm/solvers/processed_variable.py	10.65%	176 Missing ⚠️
...s/pybamm/src/pybamm/meshes/unstructured_submesh.py	80.41%	105 Missing ⚠️
...ybamm/src/pybamm/discretisations/discretisation.py	44.28%	39 Missing ⚠️
packages/pybamm/src/pybamm/meshes/meshes.py	66.25%	27 Missing ⚠️
packages/pybamm/src/pybamm/solvers/solution.py	44.44%	5 Missing ⚠️
...bamm/src/pybamm/expression_tree/unary_operators.py	80.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5688      +/-   ##
==========================================
- Coverage   98.12%   97.24%   -0.88%     
==========================================
  Files         339      341       +2     
  Lines       31996    33512    +1516     
==========================================
+ Hits        31396    32590    +1194     
- Misses        600      922     +322     

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