Coupled drainage trench benchmark

• add incoming flux to shallow water solver (#33 (closed))
• try getting rid of smooth boundary between soils
• switch between pure loam, loam + clay, pure clay (if possible due to stability)

Output:

• mean, max, min convergence factor
• convergence factor over time
• look at \max(c\cdot K), \max(c/K) in all three cases: convergence factor related to this?
• study with changing \omega: show that optimal \omega_\mathrm{opt}\approx 1
• convergence factor depending on \Delta t, \Delta z, \Delta x