Add pointers to higher-order time-stepping schemes

[BenjaminRodenberg]

Adds pointers to Niklas' and Tobias' work on solving the heat equation using implicit Runge Kutta methods and pySDC.

This is a draft, because we have to wait until #557 is merged and Tobias' thesis is published on MediaTUM.

Checklist:

• I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.
• Add pointers in source code to Tobias' thesis.

[BenjaminRodenberg]

@MakisH is there a better place to put this information? We could also add a pointer to #415 and #557 but I prioritized providing pointers to the thesis documents. The PRs are visible in the history. The link to the thesis documents is not so clear.

[MakisH]

I think this is the perfect place for this information.

[BenjaminRodenberg]

Closing this PR. Superseeded by #574. Added 0ccab4e directly to develop.