Fix interaction of temporary terms with steady_state operator

When the same complex expression appears outside and inside a steady_state()
operator, the same temporary term would be used for both cases, which was
obviously wrong.

The fix consists in never substituting temporary terms for expressions inside
the steady_state operator().

Incidentally, this implies that external functions can no longer be used inside
steady_state operators (since their computed values are stored inside temporary
terms).

(manually cherry picked from commit c27342cf)