Add conditional check for steady state in unit root models
As detailed in #573 (closed), for unit root models
options.steadystate.nocheck=1 is set, which implies that there is no check for correctness of the steady state as there are theoretically infinitely many. However, conditional on the trending (exogenous) variable, the steady state should be unique. We should thus test whether this conditional steady state is correct as suggested by @stepan-a instead of allowing to use arbitrary wrong values as supposed steady states.