Incorporate a stability check when computing the steady state

[davidrpugh]

Check whether or not the eigenvalues of the Jacobian evaluated at the putative steady state have modulus less than one when computing the model steady state. Should return np.nan for unstable steady states; and raise a warning if max eigenvalue is unity.

[davidrpugh]

This issues has tentatively been fixed. The solution to this issue created another issue: the steady state solver seems to generically converge to unstable equilibria.