Lswi episodes

What this MR does / why does DuMux need it:

Special notes for your reviewer: Adds an example program for two immiscible phase flow with 3 stages with different salinity of the brine. Each stage is divided into episodes where the input flow varies. We have used this program to successfully simulate real time experiments. All additional fluid systems and material laws are contained within lswi/dumux to avoid any confusion with existing dumux modules. I have not tested with master branch, only on releases-3.0. If you find any use for the way we have implemented processing episodes and reading everything from the input file, feel free to use is in whatever way may benefit other dumux users. I'm off on a three week vacation today, so I cannot do any more work until I'm back.

I'm not sure I've done the fork-branch-merge request correctly, but all the stuff if located within the test/porousmediumflow/2pnc/implicit/lswi directory.