Move objects to be stored in trees out of util

They are fundamental parts of the generic tree infrastructure, and the
implementations depend on `Positionable`. `Entry` was renamed to `Positioned`
to make its purpose clearer. Furthermore, the documentation of these types is
now complete.
Warn about missing docs everywhere

This makes sense now to prevent regressions as the util module is also fully
documented after moving out undocumented items.
Implement Positionable for references

This is useful if one wants to use the tree as a view performing computations
on separate data structures.
Adapt gravity example to the recent changes
Merge pull request #23 from aepsil0n/positionable_restructure

Restructure the Positionable trait