Change matplotlib to the object-oriented approach

[LuckyJosh]

An other change we discussed before was the change from the REPL-style plotting with plt. ... to the use of fig, ax = plt.subplots()

[chrbeckm]

Stefan and I did it for a workshop with PhD students, and they didn't complain, I liked it as a more scalable way. But then we need to change it everywhere, even exercises and notebooks you didn't think about beforehand...

[maxnoe]

I would be in favor of changing it. We should probably keep a small section in the end about "the old way" so people are aware when looking at the docs / stackoverflow

[LuckyJosh]

Stefan and I did it for a workshop with PhD students, and they didn't complain, I liked it as a more scalable way. But then we need to change it everywhere, even exercises and notebooks you didn't think about beforehand...

At least it is easy to search for all the place that need to be changed 😀