IniGridFactory should work on a subtree of variable name

I was about to write a tutorial for PDELab on how to use our stuff and I realized that there is some flaw in how IniGridFactory works right now. It hardcodes the section name that configuration parameters are found in to e.g. yaspgrid. That however, does not allow you to define multiple YapsGrids (preferably even of different dimensions or coordinate object type).

Instead, the factory should work exaclty like Linus work in core/dune-istl!5 (closed) does it: The factory gets the section name. Making this approach work with a static variation then requires the following:

dim = 2, 3 | expand grid
gridconfiguration = yasp2d, yasp3d | expand grid

[yasp2d]
... the section configuring a 2d grid
[yasp3d]
... the section configuring a 3d grid

[__static]
GRIDTYPE = Dune::YaspGrid<{dim}>

The C++ part then reads

Dune::IniGridFactory<GRIDTYPE> factory(initree, initree.get<std::string>("gridconfiguration"));
auto grid = factory.getGrid()

I do not see any chance to do something like this right now.

In order to implement this the factory should be constructed from initree and configuration section name.