[WIP] Documentation and major rewamp of variable naming / notation

I've added a bunch of documentation mainly for the `IMEXSweeper` and also revamped naming of most internal variables to adjust for recent mathematical notation.

We should especially discuss, whether we want this non-trivial and tricky modification of the S matrix in the showcase implementation of the IMEX sweeper. This modification is nowhere explained in the PFASST and SDC papers. It already cost a lot of time to understand for a couple of people as it wasn't documented yet. Do we really want newbies to stumble upon these in the default IMEX implementation or can we afford a few more additions per iteration?

This PR is still a WIP and I'm going to add a bunch more documentation in the next days (next week).
... this to `u` (instead of `u_cast`)?