Clean / clear all `__init__.py`

[Stannislav]

It would be good to remove all code from all __init__.py files, in particular also all __all__ statements. See for example morphoclass/data/__init__.py. This will speed up imports since unused submodule imports will no longer be done, and will also render the code structure more simple and transparent.

Ideally all __init__.py should have at most these 3 elements:

• License notice header
• Module docstring
• from __future__ import annotations (since isort is configured to automatically add this line to all .py-files)