bug in CFluidProperties::Viscosity()

[norihiro-w]

CFluidProperties::Viscosity() may not work correctly in some viscosity models which depends on some variables. Unlike the density function, it is implemented in a way that all the viscosity models access primary_variable with fixed index. However its content (or the order of actual variables) can differ depending on whether variables is set externally or not. In addition, some viscosity models are implemented to work only with MULTI_COMPONENTALFLOW, which is indeed unnecessary constrain.