feat: add support for external data variables with > 2 dimensions

[chrispcampbell]

The primary change here was to generalize the code that handles subscripted external data variables so that it works for any number of dimensions. That also helped make the code more succinct.

I extended the extdata test case to include a 3-dimensional data variable and added a VECTOR SELECT scenario that mimics something in the En-ROADS model. That uncovered a bug where SDE was not emitting a LOOKUP call when the external data variable is referenced inside a VECTOR SELECT. I fixed that in visitVar so that we use some existing code that emits a LOOKUP when a data variable is used inside VECTOR SELECT.

Fixes #45