[com1] Seperate basal shear stress from rheological stress

[JuLa96]

Other simulation programms (e.g. HEC-RAS Doc) split up the friction term in a basal (external, e.g. Manning) and a rheological (internal, e.g. Bingham) stress term. In a physical sense the seperation makes sense. Possible improvements of model qualitiy due to this modification should be assessed.