Move output variables into `.jshc`

[lucialayr]

@mzomer and me were discussing that defining output variables in the .josh file can escalate quickly as it does not really change the model so keeping track of model versions get challenging. In LPJ-GUESS, the model code had the functionality to write out all possible outputs and then in the config file they would get a boolean parameter to indicate if you want to write them out. At some point we'd want to implement something similar in Josh. I think there is a way to do it without really changing the logic of the DSL but defining it in the .josh file but putting it here in case there are smarter ways to do it.