Fix incorrect static model when “diff” operator is present

The “diff” operator was incorrectly replaced by its argument in the static
model, leading to an incorrect result for the steady state.

This is because the information contained in the “expr_node” field of the
auxiliary variables storage was not consistent across all types of auxiliary
variables: for a “diff()” operator, it would contain the argument of the
operator, instead of the full node. Hence it would not simplify to zero at the
steady state.

A similar inconsistency was also present for the “expectation()” operator,
though it was not leading to an incorrect static model.