Exempt Ramsey instruments from warning in steady_state_model block

Currently, we provide a preprocessor warning if a variable has not been set in a steady_state_model-block. E.g.

WARNING: in the 'steady_state_model' block, variable 'R' is not assigned a value

But if the user provides an instrument (ramsey_policy(instruments=(R))), the steady state file needs to be conditional on this instrument. So the warning points users to a wrong "fix".