Generalize variable names in Jacobian initialization

[juliasloan25]

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In #651, we hardcode which variables are to be stepped explicitly and which will be stepped implicitly in our Jacobian initialization. This requires listing out the names of all variables - there is probably a more generic way to do this.

Note that we must include a block in the jacobian for all prognostic variables. In the case of integrated models, this includes the prognostic variables of sub-models without any implicitly-stepped variables (e.g. soilco2 in the case of stepping SoilCanopy implicitly).