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
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".