refactor: rename stiffness operators

feat(forms): add nonlinear_diffusion_stiffness form, add nonlinear_diffusion_newton_galerkin_approximated_mass_lumped form, remove nonlinear_diffusion_newton_galerkin_asymmetric(_lumped)_part

feat: add lumping kernel to used kernels

add drafts of nonlinear diffusion newton galerkin partial operators for symmetric, asymmetric and asymmetric diagonal parts

Remove unreachable and/or unneeded branches in function_space.py

Closes #43

See merge request !36

Split function_space_impls.py into smaller files

See merge request !37

MRs 849 and 851 in HyTeG changed the name of the getMaxMagnitude()
method to getMaxdoFMagnitude() in several FEFunction classes. This
commit adapts the OperatorGenerationTest accordingly.

MRs 849 and 851 in HyTeG changed the name of the getMaxMagnitude()
method to getMaxdoFMagnitude() in several FEFunction classes. This
commit adapts the OperatorGenerationTest accordingly.

See issue #41 for details.

Adds missing return type annotation and imports HOGException.

This commit fixes issue #43 by removing three unreachable and/or unneeded
branches that apparently were connected to left-overs from an initial
treatment of manifolds.

P_0 coefficients

See merge request !33

Move frozen velocity form to work with process integrand

See merge request !34

print statement creates CI failure

Rotation matrix wrapper

See merge request !32

Fix return branch without wrapper

Fix Issue 40

Closes #40

See merge request !30

Add scaled k_divdiv form

See merge request !29