Rename NanoVDB's `tools::cuda::MergeGrids` to `TopologyUnion`

[swahtz]

To be more in line with OpenVDB's nomenclature, we should rename MergeGrids to TopologyUnion (or perhaps UnionTopology so the verb is first if that's consistent with other NanoVDB tools functions). Raised in review by @kmuseth:

This PR looks good to me, with one exception - the naming (which is arguably not part of this specefic PR so I wont block it). I would much prefer we renamed the MergeGrids to TopoloyUnion since it's more descriptive (merging of grids is ill-defined) and more consistent with the existing terminology in OpenVDB (openvdb::Grid and openvdb::Tree define topologyUnion).

Originally posted by @kmuseth in #2238 (review)