don't create NaN values when trying to convert from mg/dL

@ianjorgensen @nicolashery @jh-bate whoever wants to review. Fix and tests for the creating `NaN` when `bgInput` and/or `insulinSensitivity` are not present and units are `mg/dL`.

P.S. Also alphabetized the wizard tests to make it easier to understand what's there; one of them was there twice.