Handle case where index (subscript) appears in expression position

[chrispcampbell]

I received a Vensim model where the modeler has an equation like this (simplified):

    DimA: A1, A2 ~~|
    x[DimA] = IF THEN ELSE(DimA = A2, 1, 0) ~~|

This works fine in Vensim, but fails in SDEverywhere. We do handle the case where a dimension is used in expression position, but we don't currently handle the case where a subscript/index is used in expression position.

Note that this case is more commonly handled using separate equations (in my experience) like in the following sample, but there are probably valid use cases for subscripts in expression position, so we should fix SDE to allow the form above.

    DimA: A1, A2 ~~|
    x[A1] = 0 ~~|
    x[A2] = 1 ~~|

Or:

    DimA: A1, A2, A3 ~~|
    x[DimA] :EXCEPT: [A2] = 0 ~~|
    x[A2] = 1 ~~|