Improve PETSc integration

[max-models]

• Vectorize the vec_topetsc and mat_topetsc functions, this is by far the biggest bottleneck!

❯ python src/struphy/linear_algebra/petsc_solver_ill_conditioned_example.py
n=50000 dofs (1D Laplacian, condition number ~ O(n^2))
  cg (unpreconditioned) :  15407.14 ms, niter=49489
  petsc (cg + gamg)     :    357.01 ms, niter=16
  speedup: 43.16x
  ||x_cg - x_exact||    = 4.244e-03
  ||x_petsc - x_exact|| = 4.005e-06