Add biharmonic demo (Python and C++)

[juliusgh]

This PR adds following demos that solve the biharmonic equation using a discontinuous Galerkin method:

• A Python demo for DOLFINx based on the DOLFIN biharmonic Python demo
• A C++ demo for DOLFINx based on the DOLFIN biharmonic C++ demo

As far as I know, DOLFINx does so far not include a demo using discontinuous Galerkin methods. In my opinion this would be helpful for new users of DOLFINx and users who want to migrate old code from legacy DOLFIN.

[juliusgh]

The generated CMakeLists.txt file of the demo was ignored by .gitignore, but the build pipelines do not automatically generate the file using python cmake/scripts/generate-cmakefiles, causing the regression tests to fail. I have forced Git to add the CMakeLists.txt file. Is this a good practice or what would be the way to go here?

[juliusgh]

Hi @jhale. Thank you for taking a look! In the meantime, there has been a small change in fem::apply_lifting. I have adjusted the demo and now the regression tests work again :)