[staggered] Introduce NumEqVector for sources and Neumann BCs

See merge request !638

Feature/replace amgproperties by amgtraits

See merge request !637

The Amg properties where defined in an amg header for other discretization TypeTags.
This leads to a lot of dependencies because files from all model types where included.
Now the includes are reduced to property forward declarations and disc methods.

Fix/staggered privars

See merge request !636

* Use PrimaryVariables instead of InitialValues, SourceValues, ...

* PrimaryVariables are now only used to set Dirichlet BCs, sources and
  ICs

temp

Feature/speedup tpfa geometries

See merge request !628

We still want to be compatible with Dune 2.5 where the optional
class is not yet available.

Merge feature/2pncmin into next

See merge request !632