[2p2c] Improve Millington&Quirk and constant tau

Merged Christoph Grüninger requested to merge feature/constant-tau into master

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

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