SymbolTable::getOrigSymbIdForAuxVar() now also works on unaryOp and diffForward auxvars

This is a more natural semantics.

Incidentally, this fixes a bug in the variable mapping (M_.mapping) where some
endogenous, appearing in a log() in a VAR or TCM, would not be mentioned (e.g.
in the var-expectations/7/example1.mod test, and many others).