Lazy init yaml because canmatrix might not be fully loaded when loading this format.

def __init_yaml():
    """Lazy init yaml because canmatrix might not be fully loaded when loading this format."""
    global _yaml_initialized
    if not _yaml_initialized:
        _yaml_initialized = True
        yaml.add_constructor(u'tag:yaml.org,2002:Frame', _frame_constructor)
        yaml.add_constructor(u'tag:yaml.org,2002:Signal', _signal_constructor)
        yaml.add_representer(canmatrix.Frame, _frame_representer)