Make the book importable

[d70-t]

Through the evolution of the book, I'd expect that there will be some pages coming up which describe useful handcrafted functions which may be reusable in other parts of the book as well as in user code outside of the book. It would be great to come up with a possibility to mark parts of the code or code-cells as exported and then collect all the exported code segments into a python module which would be published to pypi.org. If that works well, one could write code like:

from how_to_eurec4a.chapter_name import nice_utility_function
nice_utility_function(my_dataset)