Added parameterized tests for empty-initialization of concrete classes.

The following now holds for all classes (but two) in the system:

    # TODO: make these classes empty-initializable
    _classes_to_ignore = (
        datastructuretools.ContextMapComponent,
        markuptools.Markup,
        )

    classes = documentationtools.list_all_abjad_classes()
    @pytest.mark.parametrize('class_', classes)
    def test___init___01(class_):
        r'''All concrete classes initialize from empty input.
        '''

        if not inspect.isabstract(class_):
            if class_ not in _classes_to_ignore:
                instance = class_()
Removed _default_positional_argument_values entirely.