write a dictionary to identify which variables are used in a specific model configuration

[martynpclark]

For example, when a model does not use a given parameter, that parameter should not be specified. Getting fancy, if a user attempts to specify a value of a model parameter that is not used (e.g., as part of calibration), SUMMA should return an error message. And SUMMA should not output variables that are not used.

[bartnijssen]

This will become increasingly important as we expand the number of physics options and the size of the domain that we run over