Finally seems to stop mixing up material numbers for every other resolution I try it out with.
Cylinder diameter is now actually set to 0.1m as called for by [SchaeferTurek96].

Mainly to further declutter simulation and refinement setup

This allows for readable differentiation between constructor overloads.

By deriving RefiningGrid2D from Grid2D to provide methods specific to refining subgrids.

Scaling factor is specific to the refinement method implemented by the coupler.

Works but is nowhere near anything one could consider good.
Obvious issues:

* More than one cuboid per grid makes it harder to determine the next lattice cell to be coupled
	* i.e. currently lattice positions are determined ad hoc by resolving their physical position
* Coupling is not actually parallelized
	* All coupling lines are traversed by all processes, way to much communication
* Load balancing and cuboid decomposition doesn't care about refinement
	* ideally refined cuboids should be computationally near their coarse _parent_ cuboids

The first two isses should be fixable with a reasonable amount of work.
This sadly doesn't apply in any form to the last issue.

Grid refinement is at its easiest when discrete materials are set using
analytical - i.e. resolution independent - indicators.

Coupler construction is to be wrapped by `Grid::refine` for refinement
areas that are fully inside the coarse domain.

Starting point for integration into a more flexible setting.

See "Advances in multi-domain lattice Boltzmann grid refinement" (2012)

i.e. not only the moments and non-equilibrium parts.

Looks surprisingly well from a purely visual perspective.

Coupling overlap of one coarse grid width.
Coarse grid points intersect fine grid points in this area.

i.e. restrict to non-forced BGK dynamics and bounce back boundaries.
Remove further distractions that will have to be modified anyway such as error norms.