fix: handle subdimensions correctly for GET DIRECT CONSTANTS

[ToddFincannon]

Fixes #124
Fixes #134

Details in issue #124.

(Edit by Chris: This also resolves the regression reported in #134, so I've included my except test modifications on this branch so that we have additional test coverage for that particular case, which did not use GET DIRECT CONSTANTS.)

[chrispcampbell]

@ToddFincannon I verified that both En-ROADS and C-ROADS are able to be built with this branch (it resolves the En-ROADS regression that I reported in #134). I included the tests I wrote for that issue in this branch for additional coverage.

I'm trusting that the changes here resolve the issues at hand since the tests all pass, but I wish I had a better understanding of the subscript/dimension handling code in SDE (and how they're handled by Vensim in general). I'm keen to (someday) tease some of the subscript handling code out into separate modules that can be more precisely unit tested, which I think would help make it easier for me to understand how these pieces fit together, but that's for another day. In the meantime, I'll get this merged and will continue with the other PRs.