#220 Allow plotting of systems with multiple channels

[Ulf Lorenz]

What and Why

Currently, there are only plot drawers that support plotting of one-dimensional systems. However, an important class of systems has two dimensions, where one are coupled channels (electronic states). Augment the 1D plotting to be able to handle 2D systems where one is a coupled channel. Equivalently, with [#211] implemented, allow 2D plotting of 3D systems, where one is a coupled channel.

There is a lot of additional infrastructure required. For example, we need a projection onto each coupled channel, have a set of 1D potentials instead of a 1D potential, ...

Possibly we want a new drawer that can deal with this specific use-case, or we augment the current drawers such that the 1D case is just a special case with one channel or so.

Acceptance criteria

• There is at least one demo (1D + coupled channels) that demonstrates the plotting of the wavefunction on multiple channels.