Repurpose passenger and freight subscripts for aircraft and ships as "domestic" and "international"

[mkmahajan]

The domestic and international designations are a more important distinction than passenger v. freight for these vehicle types, and most questions we get asked about these vehicles revolve around separating out the international component.

[jrissman]

This is a great idea!

I know this can be done as data-only (and relabeled in the web app output graphs), but we might want to change the official subscript element names in Vensim for a future release, because this is such a good idea that it should probably be the default.

[robbieorvis]

Consider letting vehicles carry both passengers and freight in the same vehicle in addition to other proposed changes.

[jrissman]

We might add a new domestic/international subscript rather than repurposing the passenger/freight subscript, as passenger/freight could still be useful.