Multidomain wrappers to easier write main files
- wrapper for grid variables for multidomain (when it's more than two it already gets nasty in the main file)
- wrapper for vtk output module
The same could be done for problems, grid geometry. I think, there is more such classes it would get even shorter as all the type aliases are hidden too.
It gives nasty compiler errors though, if e.g. you put in the wrong type the message is crazy long because of those Hybrid::forEaches.