Allow for more control over which context graphs are displayed for a scenario or view

[chrispcampbell]

We use model-check extensively when developing En-ROADS. We configure model-check to include all graphs that appear in the main En-ROADS interface, but we also started including special comparison graphs that compare En-ROADS outputs to historical datasets or the outputs of other models.

There are some limitations in model-check that we need to address to make this more usable for our team:

• Currently model-check doesn't make it easy or possible to put graphs into different buckets, so right now all of the app graphs are getting displayed in the same grouping as the comparison graphs. It would be better if we could declare "groups" of graphs, similar to what we have with scenario groups.
• Currently the "grouped by diffs" view of the graphs is only available if you use the special "graphs: all" preset. We would like to be able to explicitly request this kind of view when specifying any set of graphs (whether with an array of graph IDs, or by referencing a graph group).
• Currently there is no way to filter out context graphs that are unrelated to a particular scenario. We have special comparison scenarios defined, and there are certain comparison graphs that are only relevant for one specific scenario, so we should be able to configure this.

[chrispcampbell]

The first two items are interrelated and will be addressed together, but the last one is separable so I moved that to a new issue #540.