Access values stored in the acceleration data structure

[dalg24]

Add (non-modifying) access to the values stored in the BVH and BF

Opening PR early to get feedback.
In the current version the test is meant to check that we can retrieve the values and that users shall not rely on the order in which they are stored in the data acceleration structure.

[aprokop]

For BruteForce, everything is straightforward.

For BVH, though, the semantics are interesting. What we are saying, essentially, is that we can get a value by index. But we don't know which one. We can guarantee that spanning the set of indices a user will retrieve the set of the original values.
So, the question is, what benefit would a user have from having this operator?

In some sense, our container is similar to std::set, where we can ask and answer questions about whether a specific element belongs to a container, iterate over elements, but not access a specific element without calling find first.

The situation would have been different if we could isolate values from nodes. In some situations, with heavy values, we could probably do that. For light values, though, I think the performance suffer.

Boost rtree does not have value accessor, only begin() and end() that could be used to go through the stored values.

[aprokop]

Closed for no action and no plan. Feel free to reopen if there is a way to proceed.

[dalg24]

I think access to values can be useful regardless of the order but I am fine to wait for an actual ask before we provide it.