Soulsby slope effect for all threshold formulas

Soulsby slope effect (SLOPEFF=2) is only activated for Meyer-Peter Mueller formula but should be done for all threshold formulas (Rijn (7), Hunziker (6), Soulsby (5), Dibjana (9) and Wilcock&Crowe (10)). For some of the formulas it is not critical Shields, but tau_crit or u_crit, ... For these formula a calculation Shields-crit/u_crit (or others) is needed. Additionally a stop and message should be programmed in lecdon in case of choosing Soulsby slope effect and a non-threshold formula.