This saves us a couple of lines in the `load_patient_data` method and is
more readable.

Some callback functionality that was tested in a pre-release has been
forgotten in the code base and is now deleted.

The `to_dict()` method returned a wrong graph dictionary when trinary
due to growth edges. This is fixed now.

The `get_mermaid()` and `get_mermaid_url()` methods now accept arguments
that allow some modifications of the output.

The marginalization should only be skipped (and 1 returned), when the
entire disease state of interest is `None`. In the midline case, this
disease state includes the midline extension.

Previously, only the involvement pattern was checked. Now, the model is
more careful about when to take shortcuts.