'Loads the proper action by the type attribute of elem'

@classmethod
def from_elem(cls, parent, elem):

    if (elem is None):
        option_type = NullToolOutputActionOption.tag
    else:
        option_type = elem.get('type', None)
    assert (option_type is not None), "Required    'type'    attribute    missing    from    ToolOutputActionOption"
    return option_types[option_type](parent, elem)
