Improve staggered free flow model

• Account for extrusion factor (see !914 (merged))

• fluxes

• storage terms

• Add 2D friction term to mimic 3D flow (see !925 (merged))

• Implement better tests (e.g. 3D, transient, backward-facing step, etc.) (see #415 (closed))

• Improve the documentation (help message) and appearance of the test provlems

• Improve inheritance structure (especially for RANS models) (see !915 (merged))

• Improve boundary conditions (#487 (closed))

• Check caching on/off