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 The notebook previously suggested installation from PyPi, but assimulo is outdated there (3.0 from 2018) and does not build any longer.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.

[jessica-mitchell]

The notebook works! Thanks @heplesser

[jougs]

Phew... Honestly, I don't like the fact that this adds a dependency on conda to build the full docs. I have now contacted the main developer of Assimulo and asked for an update of the PyPI package and would like to keep this un-merged until I get an answer from him.

[heplesser]

@jougs Why do you consider a conda-dependency more problematic than a PyPI dependency? That said, I am wondering if it could make sense to move background documents such as this to a separate repository to avoid too many dependencies in the NEST code base itself.

[jougs]

I always feel like conda is the odd one out, because it is not installable/updatable via my system's trusted repository, while pip is. I'm usually trying to avoid either if I can, but assimulo is also not available via apt-get...

Splitting the documentation would not be a good idea in my view, as that has a high potential of things getting outdated and forgotten.

Thinking about it again, I realize that this is only a run-time dependency, and I can probably live well with it being mentioned in the environment.yaml, which I'm anyway not using. I'm thus approving.