Convert the plant_uml example into a utility function that is part of the SML library

[abrownsword]

Looking at the latest plant_uml example code, I think it is safe to say that this goes beyond being simple. This is a very useful capability, and it would make sense to have the plant_uml example actually turned into a function available in the SML library. Ideally it would take an ostream& parameter so the output can be redirected.