auxiliary variables in steady state and static model

Currently, in static model and set_auxiliary_variables.m, auxiliary variables may appear in the RHS of equations. This is problematic for two reasons:

1. assignments may not be recurice in set_auxiliary_variables.m which leads to errors
2. _static.m may not be solved independently from set_auxiliary_variables.m

In the static deterministic model, any auxiliary variable, except Lagrange multipliers for Ramsey policy, can be replaced with an expression using only original variables.

In the preprocessor, I want to rewrite _static.m and _set_auxiliary_variables.m along these lines.

I expect this changes to solve #1119 (closed) and #633 (closed)