Add support for cin files

[chrispcampbell]

A colleague at CI has suggested that SDE should support reading Vensim's "cin" file format as a way to override constant initializations in the model.

This would be useful in scenarios where a modeler wants to experiment with different initial values for variables (in Vensim, using cin files) and then be able to build the same model in SDE without having to edit the original definitions of those variables in the mdl files.