[octree] Get neighboring cubes

[movabo]

Implementing the neighbors method requested in #190 (with a slightly different method signature).
I think we should wait for #368, #262, and #342 before merging this.
Still, it would be great already get feedback on the code. :)
I didn't check it on clang-tidy right now because I expect a lot of changes in this regard with #342.

[IAmNotHanni]

Good work!

[IAmNotHanni]

Nice work! Just some small things.

[IceflowRE]

I will look into this in the next days, i need some more time for this.
Just for now, iam not a huge fan of the cached index value.

[movabo]

^{Summarizing what I already wrote on Discord:}
I'm not a fan of it either. However I could not come up with a solution without knowing the index.
And then, I think it's better to keep track of the index than again-and-again iterating over the children-array of the parent to find the index.
Also, my guess is that it might be worth to save the index in a variable for the future.

[yeetari]

Nice work. I'm not super sure on the octree code, but it's always still fun to review it. One thing to keep in mind though, and this is probably more of a concern for the future, is that having lambdas/heap allocations/lots of shared_ptr copies in hot code (which I think the octree is/will become since it's at the core of the engine) is generally a bad idea. Having said that, if this function isn't called very often (its main use is for the octree editor or whatever), then it's fine as is.

[IceflowRE]

I suggest to squash.