Investigate potential bug introduced by 15716124

The change introduced in 15716124 was meant to fix the existing bug in aux_equations that did not recursively substitute variables in aux_equations even though they were substituted in equations. We must see if this commit introduced a bug or if there is a bug in the way ramsey code deals with auxiliary equations.

@JohannesPfeifer pointed this out due to the failure of optimal_policy/Ramsey/ramsey_ex_aux.mod introduced by this commit