Install assimulo from conda-forge

[heplesser]

The aeif_models_implementation requires the assimulo package to compute reference solutions. The notebook previously suggested installation from PyPi, but assimulo is outdated there (3.0 from 2018) and does not build any longer. A current version is available on conda-forge. This PR therefore adds assimulo to environments.yml and updates the requirements information in the notebook.

@jougs @terhorstd Since this introduces yet another dependency for a corner case, I'd like your comments.

Fixes #2454.