Add style property to change shape of rendered voxels

[jacquelinekay]

Since rendering boxes in Rviz has poor performance when scaled to very large octomaps, I exposed the render mode of individual voxels in the octomap Rviz plugin in order to use a more performant shape (e.g. squares or points).

[ahornung]

Are all the non-boxes variants also scaled properly, do they fill the correct volume? Or are they just at the center like the MarkerArray display in RViz?

This could lead to a wrong impression for e.g. pruned inner nodes. That's OK but could be pointed out in the description (e.g. "Points (center only)").