Add HydPy-WQ-Widths #173

We recently implemented HydPy-WQ-Widths-Strickler, and it appears consistent to also implement its sibling HydPy-WQ-Widths similar to how we implemented HydPy-WQ-Trapeze-Strickler and HydPy-WQ-Trapeze. However, the only model family that provides main models that could use HydPy-WQ-Widths is HydPy-SW1D, and none of its main models could use the central feature to distinguish between those parts of a cross-section that are actively involved in water routing and those that are not. So, implementing HydPy-WQ-Widths should likely be accompanied by modifying HydPy-SW1D and eventually also by extending the related interface definitions.

Generally, this seems worth the effort, considering that HydPy-SW1D is currently the most hydrodynamic routing approach available in HydPy, but it could take a few days of work. And I am not even sure if the special LIAS equation is compatible with such a "no-flow area" concept, so success is not guaranteed.