Semi-automatically populate __all__ in __init__.py files

[vpratz]

This PR implements one possible solution to the problem highlighted in #290. By specifying __all__ in the relevant files, it makes explicit which modules and functions should be documented, so that only desired functions and modules are included. This also allows setting autosummary_imported_members = False, which greatly speeds up building the documentation.

It requires manual action when:

• a new public submodule is added: has to be added to the include_submodules list in the parent module
• a new __init__.py is added: must add the following boilerplate at the end of the file (number of dots before utils depends on the path):

from ..utils._docs import _add_imports_to_all

_add_imports_to_all(include_modules=[])

Please give feedback on the underlying mechanism. Is this an option we would be happy with, or should we rather try something else?