Remove OctreeMeta's Default instance

[nnmm]

This is to make https://github.com/googlecartographer/point_cloud_viewer/pull/415 smaller.

This makes the two dangerous/unexpected parts more explicit and doesn't allow them to hide in ..Default::default(), namely:

• You shouldn't use a zero AABB as the bounding box for generation, but the Default instance gave you such an AABB.
• The two magic attributes "color" and "intensity".

The attribute data types are duplicated in two places now, but that's necessary for moving towards a state where octrees can have more attributes: The occurence in build_octree() will be removed, while the one in from_data_provider() will stay.

In the end, it's a matter of taste.

[nnmm]

meow @feuerste

[feuerste]

Having the Default implementation actually served the purpose of getting rid of redundant code. How about providing a new(resolution: f64, bounding_box: Aabb3<f64>) method instead?

[nnmm]

Would it be fine to make that HashMap a constant as well?

[nnmm]

Ok, we don't have lazy_static as a dependency, I'll go with the new method.

[feuerste]

Using lazy_static and a const hash map is fine with me, too, whatever you prefer...