OcTree: Plot grid for certain level

[Dieter Werthmüller]

I think it would be nice to plot a certain level of the OcTree,

mesh.plotGrid(level=2)

or up to a certain level

mesh.plotGrid(max_level=3)

Not sure how straight forward it is to implement though.

[Dieter Werthmüller]

Here a made-up examples for different levels.

Thinking about it, having level and max_level probably doesn't make sense. Only level, but up to that.