Due to !1905

Timo Koch authored 5 years ago and Ned Coltman committed 5 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

* With this patch you only include what you need
* The header gridmanager.hh includes all specializations
  as convenience. This makes this patch also fully backwards-compatible