ADR 001: __init__.py for documentation

We have decided to include __init__.py although our code should be able to exploit the implicit-package functionality of Python. This is so as to make sphinx-autoapi work well; the __init__.py files enable the package structure to be recognized correctly. As a byproduct, we are able to write the documentation frontpages of the API reference for each package – we will be using the __init__.py mostly for writing docstring there.