Handle model values

[will62794]

Decide how to handle "model values" for instantiation of CONSTANT parameters. Currently, it is expected that string values are to be used in place of model values.

[will62794]

Partial support in ca84c8e.

May still need some additional logic to handle nested sets of model values e.g., Quorum = {{n1},{n2},{n1,n2}}

[will62794]

Parsing of general model values should now be handled after ffe5360. See example Paxos spec + model here.