[2p2c] Improve Millington&Quirk and constant tau
A quick test (Clang 3.8, one run, no error bar) of test_box2p2c with the optim.ops from dumux and a 100 times finer grid:
Millington&Quirk: 99 s
Constant Tau: 95 s
That's about 5%. But more measurements are required and the differences in the solution must be checked.
This merge request does not address all changes proposed in the task FS#321