Correct implementation of wave front propagation

src/bvpy/ibvp.py

src/bvpy/solvers/nonlinear.py

src/bvpy/solvers/petsc_solver.py

src/bvpy/solvers/time_integration.py

src/bvpy/vforms/transport.py

tutorials/.gitattributes

tutorials/bvpy_tutorial_7_hyper_elasticity.ipynb