Stopping point for JSON serialization. Will now bring class names in line with PAT.

[#53261073]
Renaming "Perturbation" to "Measure".

[#53967295]
Step 1 for making MeasureGroup class.

[#53967295]
Moved DiscreteVariable functionality into MeasureGroup. Made DiscreteVariable be an abstract base class with the notion of valid integer values.

[#53967295]
Initial tests and json output.

[#53261073]