Closes #3452

I decided to just add __init__ methods to the empty classes that call self._freeze(). That requires fewer changes than setting self._freeze = True in TO.__init__ and using with self._unfrozen() in all the classes …

Checklist for PRs

• [ ] Added .. versionadded:: version, .. versionchanged:: version or .. deprecated:: version to the docstrings for user facing changes (for methods/class descriptions, arguments and attributes)
• Created new or adapted existing unit tests
• Documented code changes according to the CSI standard
• [ ] Added myself alphabetically to AUTHORS.rst (optional)
• [ ] Added new classes & modules to the docs and all suitable __all__ s