Refactoring of the forecast tutorial

[emanuel-schmid]

Changes proposed in this PR:

• This is Refactoring of the forecast tutorial #1032 with the develop branch as target

This PR fixes #

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[chahank]

It looks like together with the typo correction, all the plot have been regenerated. This creates quite a lot of changes in the git log. Do we want this?

Otherwise good to go. I think neither the pre-commit hook fnor petals compatibility failing is related to this PR.

[emanuel-schmid]

@chahank yeah. right. The problem is that every plot change adds to the size of the repository in the range of MBytes.
In the long run this leads to a download size for the whole repository that is just not bearable anymore.

@elianekobler - I know that changing only the relevant lines in a notebook is a bit tricky, but still ...