Default values for constants

Currently, a constant in Modest and JANI either has a value assigned in the model, or it is open - then a value has to be specified on the command line for all tools in the Modest Toolset (or one could use a parametric analysis with e.g. Storm). It would be convenient to be able to define a "default value" for (some) constants in a model, so that they take this value unless one is specified on the command line. This would be an extension to both Modest and JANI.