Move semantics for Grid (and Tree?)

[BenFrantzDale]

As far as I can tell, openvdb::Grid holds a std::shared_ptr<Tree>. I haven't figured out why the tree needs to be shared (shared non-const is scary!) but at very least that means Grid should be movable.

Describe the solution you'd like

I think this is just a matter of

GridBase(GridBase&&) = default;
GridBase& operator=(GridBase&&) = default;

and

Grid(Grid&&) = default;
Grid& operator=(Grid&&) = default;

I suspect TreeBase, Tree, RootNode, and InternalNode could get the same treatment. (LeafNode too although that only matters if people are storing things in leaves that allocate on the heap, so that's probably silly.)

Describe alternatives you've considered

At present, the use pattern seems to be very focused on shared pointers to mutable structures. (I've been watching a bunch of Sean Parent talks recently, and he rants about shared_ptr to mutable, particularly in the context of reasoning about thread safety.) There are lots of places (I think!) that I could avoid the extra allocation and get the value semantics and thread safety of a regular type.

Would y'all be open to a PR making the above additions?