Subscript Aggregation

[JamesPHoughton]

Vensim allows you to perform certain aggregating operations across dimensions of a subscripted variable, such as:

SUM(My Variable[Dim1, Dim2!]

We'll need to modify the parsing grammar to handle this, and properly construct sum, product operations, etc.

For an example, see: https://github.com/SDXorg/test-models/tree/master/tests/subscript_aggregation, which should serve as a test case for this functionality.