Error in non-Newtonian Casson model for small shear rates

In the non-Newtonian Casson model, implemented in src/lbmpy/non_newtonian_models.py, the adapted omega is only valid for theta < 1 (cf. https://doi.org/10.1007/s10955-005-8415-x, text after eq. (14)). In particular, for small shear rates where theta >= 1, the new value might be NaN due to a negative argument in sqrt. I suggest to fix this by replacing line 64 by

omega_cond = sp.Piecewise((omega_min, (new_omega < omega_min or theta >= one)), (omega_max, new_omega > omega_max), (new_omega, True))

This means, that the relaxation rate is at least omega_min, i.e., the maximal viscosity is limited, as intended in the original publication.