Wall-stress model verification

The wall-stress model (i.e., the stand-alone function that maps the LES solution at height \(h_{\rm wm}\) to the wall stress and other wall quantities in the wall-stress modeling approach) is a highly encapsulated entity that takes very few inputs and produces very few outputs. As such, it is ideally suited to a highly encapsulated implementation (in a class, or module, or function). This also means that the wall-stress model itself should be verified prior to testing the full WMLES code.

To be added: a few verification cases.