Divergence and Gradient Forms for Conforming Crouzeix-Raviart element in 2D

See merge request !47

The level of nesting in the if-statements apparently was too much
for mypy to understand.

Notes:
- The functionality is required for issue #49.
- Generation of DG-DG operators currently seems to be a more theoretical
  perspective. See issue #19.

Status:

- The high level part, such as form definition and name generation works.
- We can successfully generate fro triangles the following forms:
  * dg1_to_p2_plus_bubble_divt_affine_q3
  * dg1_to_p2_plus_bubble_divt_blending_q3
  * p2_plus_bubble_to_dg1_div_affine_q3
  * p2_plus_bubble_to_dg1_div_blending_q3

The commit adds a DGFunctionSpace to represent discontinuous piecewise
polynomial functions a.k.a. Discontinuous Galerkin. Currently it supports
degrees 1 and 2 in 2D and 3D.

Update location of header P2PlusBubbleFunction.hpp in HyTeG

See merge request !46

Following MR hyteg!887 the header file P2PlusBubbleFunction.hpp
no longer resides in experimental, but in p2functionspace.

Clean up indexing, Pt. 1

Closes #46

See merge request !43

Give hint to user on the need to extend generate.py

Closes #53

See merge request !45

Fixes #53

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

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.