[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).
The second half of this sentence isn't quite right.  Typically when implementing a backward-Euler stepper and/or a multistep BDF type scheme, you/the user has to provide a solver to solve `U(n+1) - dt F(U(n+1)) = RHS` for `U(n+1)`.  In SDC land we (depending on the nodes used) also sometimes to just evaluate `F(U(n))`.  Note the difference: in one we have to solve for `U(n+1)`, in the other we just evaluate at `U(n)`.  So... how about:

This sweeper requires three interfaces to be implement for such ODEs: two routines to evaluate the explicit \\(F_{\\rm expl}\\) and implicit \\(F_{\\rm impl}\\) pieces for a given state, and one that solves (perhaps with an external solver) the backward-Euler equation \\(U^{n+1} - \\Delta t F_{\\rm impl}(U^{n+1}) = RHS\\) for \\(U^{n+1}\\).

Also, spelling: 'effort' instead of 'efford'.