M. Mandement 24/01/2022:bugfix : init of theta_l in mixtures was too low (0.1K) ==> 300K

This resolves the bug trigged in th_r_from_thl_rt_1d.f90 at the LOG(ZT) when some HALO points has T values < 0.
TODO: the computation in compute_entr_detr should be done only on updraft points