Modify the bed shear stress calculation within the shallow water friction laws.

The function shearStress returns the bed shear stress normalized by the water density. This normalisation is unfavourable, since the name of the function (and also the documentation) does not reflect it. Also the name shearStress itself is confusing, since the bed shear stress is not a classical shear stress tensor. The name bedShearStress is more clear, since it points to the context of shallow water equations.

TODOs:

• Modify the function shearStress to return the bed shear stress and not the bed shear stress normalized by the water density.
• Return the positive shear stress
• Rename shearStress to bedShearStress
• Improve docs