Provide simple way of specifying compartments

[mhucka]

People sometimes have more than one compartment in a model. An easy way to handle this in the interface would be to let the user specify the Matlab variables that are meant to be compartments. The command line interface could have a flag letting the user list the compartments,, and the GUI could provide a text box where users can write the variables.

Eventually, we will want to add the ability to use comments in the Matlab code to define compartments, and also to provide a way to list such things in a separate file. For now, just having a way to do it in the UI would be useful in itself.