Fix pyproject.toml for wheel generation

See merge request !44

The packages list was missing the new module introduced in !37 [c3815303].
Commit adds hog.operator_generation.function_space_implementations to list.
This is a recurring problem, see also !22 in this respect.

Resolve issue 48

Closes #48

See merge request !42

This should temporarily fix the problem in the "form-generation" job of the
pipeline with the p2_plus_bubble_diffusion_affine_qe form. For the moment we
place it in the p2 folder. However, file_path() needs refactoring, especially
if we add more spaces.

The commit
- adds component_index as keyword to IntegrandSymbols
- makes gradient and divergence form pass component_index to process_integrand
- the corresponding integrands now check the shape of grad_u resp. grad_v
  and, if it is not square, revert to extracting the corresponding component
- moves the is_implemented check from the call of HyTeGIntegrator outwards,
  so that the call to form_func is protected against out-of-bounds indices

Add support for bubble enhanced P2 element in 2D

See merge request !40

Make LagrangianFunctionSpace.is_continuous return False for P0

See merge request !41

Note that we return False for is_affine(), because there seems to be a problem
with the implementation of the optimisation based on this for True.

Note that this commit includes an IMHO rather not so nice hack for
the facedof_index function. The latter assumes that index is
Tuple[int, int, int]. However, the index it receives contains only
two entries.

Note that the ordering of the shape functions associated with the edge dofs
needs to be the one used by FEniCS/HyTeG.

Note that this is only for testing form generation and verification of the
result. The generated forms are, currently, not useable in HyTeG as there
is neither a base class for them nor any operator that could make use of
them.

This space represents consinous quadratic Lagrange elements on simplices
extended by a bubble function. The current implementation is only for 2D
triangles.

Adapt integration tests and CI to HyTeG's new CMake configuration

See merge request !38