SmoothingOperator: do we really need inverse_times() and friends?
Some of the current regresseion tests show divisions by zero when calling inverse_times() because exp(-x*x) for x around 20-30 bacomes zero.
If there is no need for this functionality in real-word code, I'd suggest of disabling the problematic methods. If that's not possible, I'd encourage testing for underflow and raising an exception if it occurs.