#178 Add Wigner transformations and -plots and reduced density matrices

[Ulf Lorenz]

What and Why

Followup of [#155], [#165]. Now that we have transformations between different representations, implement what we need to enable the nicer plots found in the Matlab version.

In particular, we need a Wigner transformation as a special transformation, and a reduction to a single degree of freedom, i.e., tracing out other degrees of freedom. With these two tools in our box, we can implement advanced plotting, in particular the nice-looking 1D-plots of the Matlab version, and a plot for higher-dimensional systems.

This issue is important for convergence checks. Basically, only by looking at the (at least reduced) density in DVR and FBR can we study the convergence of the calculations.