Simplify time stepping schemes and add type hints and documentation

[BenjaminRodenberg]

• Remove support for monolithic case (with vector valued displacement and velocity) from time stepping schemes. This is not needed and removing this feature considerably simplifies the code.
• Improve readability by using standard names (e.g. stages are $k_i$ in literature, not $s$).
• Add type hints and make sure mypy works without problems.

Checklist:

• I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md. no user-facing changes
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.