VECTOR ELM MAP with dimension name in offset generates incorrect code

[ToddFincannon]

When a subscript name appears as a constant for the offset in VECTOR ELM MAP:

DimA: A1, A2, A3 ~~|
DimX : one, two, three, four, five ~~|
x[DimX] = 1, 2, 3, 4, 5 ~~|
y[DimA] = VECTOR ELM MAP(x[three], (DimA - 1)) ~~|

the offset appears in the wrong place in the generated code:

for (size_t i = 0; i < 3; i++) {
  _y[i] = ((i + 1) - 1.0)_x[_dimx[2 + ]];
}

[ToddFincannon]

To fix this we needed to detect when we are on the second argument of VECTOR ELM MAP and emit expression code to vemOffset instead of directly into the expression channel. Then it can be inserted into the RHS expression that replaces the Vensim function when the rest of the parts that go into it are ready.