Coupling methox from Koch et al 2020 JCP (https://doi.org/10.1016/j.jcp.2020.109370)

Replace calls to deprecated `DUNE_VERSION_NEWER_REV(DUNE_ISTL,2,7,1)`
by `DUNE_VERSION_GT_REV(DUNE_ISTL,2,7,0)` which also reads better than
the `GTE` version, as Dune 2.7.1 hasn't been released yet. Fix the
call to `DUNE_VERSION_GT(DUNE_ISTL,2,7)` in the solver factory
backend which would break things in case of a 2.7.1 release. Adapt
the corresponding changelog entry.

Unfortunately the CMake logic for quad math detection was overwriting the Quadmath
package from Dune and had to be removed. This removes all quadmath code from
Dumux and uses the quadmath implementation of Dune instead (Dune::Float128)
which is based on the same library.

Due to !1905

Timo Koch authored 4 years ago and Ned Coltman committed 4 years ago

FluidSystem::getMainComponent returns the main componet index of the phase.
This doesn't actually make sense for the tracer models which why this commit also
* removes getMainComponent for tracer fluid systems (Changelog amended)
* guards all occurences of getMainComponent in code that works for tracer too
with constexpr if