First iteration on creating the octopus tutorials with postopus

Goals:

• Try to replicate the octopus tutorials with jupyter notebooks and postopus, without worrying about the text, but just running the commands.
• We should run everything again in the notebooks, because the tutorials and the version of octopus may have been updated. Useful magic command: %%file
• If there are features missing for replicating the tutorials, open an issue to discuss it and implement it if necessary. ( - Delete the computed tutorial data from the postopus-testdata repo if we don't need it)
• Implement binder with some of the tutorials?