Vel factory function

def create(backbone: ModelFactory, vmin: float, vmax: float, atoms: int,
           input_block: typing.Optional[ModelFactory]=None):
    """ Vel factory function """
    if input_block is None:
        input_block = IdentityFactory()

    return QDistributionalModelFactory(
        input_block=input_block, backbone=backbone,
        vmin=vmin,
        vmax=vmax,
        atoms=atoms
    )