Clean / clear all `__init__.py`

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)