generatorCode can now have arg names that are the actual requirements.

So we get
   > extractRequirements(clearanceRate)
   [1] "Multiplier"
and also
   > evaluateOutput(clearanceRate, 10)
works!
evaluateOutput now works whether or not an argument is provided.
OK, scoping is fixed, evaluateOutput(clearanceRate) is now working.  Returns a VariableValue. Next, add ability to add the requirements to the environment.
Using print method defined in VariableGenerator.R --- Typically we will NOT assign the return value to an R object, except temporarily in evaluateOutput().
Merge branch 'try-reqs-in-generatorCode-arg' into reqs-provs