stiff_lambda_test:

  Calling with no arguments returns the default:
    lambda_output = stiff_lambda ( ) =  4.0

  Calling with an argument resets the default:
    stiff_lambda ( 99.0 )

  Calling with no arguments returns the (new) default:
    lambda_output = stiff_lambda ( ) =  99.0

  Calling with input and output returns the new default:
    lambda_output = stiff_lambda ( 33 ) =  33.0