GitLab

Add missing remarks from Alexander (they have been lost somehow)
Add last minute remark of Monika

As discussed with Daniel, it might lead to confusion if you need to
change `krotov.functionals.J_T/chis_ss` to `J_T/chis_subspace`.
I directly imported the relevant parts to the namespace.

Also adjust 3lvl notebook accordingly

+ add optimization-output to cell of optimization

- Write extra remark to view the hints and solution, if they succeed in
ex0.
- add extra note to re-evalutate the hint cell to unravel more hints.
- Change failure header
- adjust ex2b to give a "it's getting warmer" solution if they are
getting closer
- add default value to ex2b

changed controllability script according to the first testing discussion. Improved the links between the notebooks and made jupyter lab standard instead of jupyter notebook.

Ignore .ipynb-checkpoints/ and __pycache__/

+ Add general remarks on how to use the notebooks (add/remove cells)