Make flatten command fail if conflicting subscripted/const equations are defined

[chrispcampbell]

Currently the flatten command will produce warnings and errors if differing/conflicting definitions of an equation are detected in different submodel files. However, we ran into a case where Vensim left an orphan definition (that was a simple, non-subscripted constant), and this conflicted with the newer definition in a separate mdl file (that was a subscripted constant), but the sde flatten command did not flag this as an error.

Example:

# in submodel1.mdl
A = 1 ~~|

# in submodel2.mdl
A[DimA] = 1 ~~|