Added a dedicated P2Vector epsilon op blending test in 2D (testing combination vectorspace + blending).

Lowering quadrature degree of vector epsilon operator in HyTeG integration test to match that of the reference operator. Otherwise, the comparison tests do not pass.

mypy is driving me insane.

mypy is driving me insane.

Trying to satisfy the mypy type checker for the dimensions parameter in the operator generation script.

Adding option to only generate 2D or 3D part in script. Then removed VectorEpsilon test in 3D since it generates very long in CI.

Using re.fullmatch also for the FE spaces in the generate_all_operators.py script. Otherwise, e.g., 'P1Vector' is also chosen if only 'P1' is specified.

Instead of having dedicated forms, the diffusion form has been modified slightly by replacing the dot product with a double contraction. This should ensure compatibility with the scalar version.

* only P1
* only 2D (3D should be straightforward)
* only briefly tested vector Laplace
* some cleanup pending

Cherry-picked be7e45e5bb08baf12bb18edfafbfc6052062c5c0 from old version: Draft for an updated version of the TensorialVectorSpace as a first step towards vector-vector operators.

Added preprint to README.

See merge request !1

To restore the closed source history:
```
git remote add history git@i10git.cs.fau.de:terraneo/hyteg-form-generator.git
git fetch history 'refs/replace/*:refs/replace/*'
```