There was one functional issue with the previous API which was its
use in `VolumeGrid<T>::grid_for_write(tree_token)`. The issue was
that the tree token had to be received before the grid is accessed.
However, this `grid_for_write` method might create a copy of the
`VolumeGridData` internally and if it does, the passed in `tree_token`
corresponds to the wrong tree.
The solution is to output the token as part of the method. This has two
additional benefits:
* The API is more safe, because one can't pass an r-value into the methods
anymore. This generally shouldn't be done, because the token should
live at least as long as the OpenVDB tree is used and shouldn't be freed
immediatly.
* The API is a bit simpler, because it's not necessary to call the
`VolumeGrid.tree_access_token()` method anymore.
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