Implement the AbstractTrees interface

[asinghvi17]

This is currently in an extension, but we could also move it into the package proper. AbstractTrees is a zero-dependency, super lightweight package.

The code here simply implements the AbstractTrees.jl interface for STRtree, STRNode, and STRLeafNode. This allows you to use AbstractTrees iterators on STRtree structures without any fuss, like AbstractTrees.Leaves, AbstractTrees.PreOrderDFS, etc.

I personally use this to extract all tiles below a certain size (or leaf nodes), and then extract the indices they contain. Such nodes might be close to or far away from the root node.

Some example code:

"""
    extract_idxs(node)

Extract the indices of the geometries that an `STRNode` covers.

Returns a vector of integers that are indices of the geometries that made the tree.
"""
function extract_idxs(node::Union{STRtree, STRNode, STRLeafNode})::Vector{Int}
    return mapreduce(Base.Fix2(getproperty, :indices), vcat, AbstractTrees.Leaves(node))
end

[maxfreu]

Hey! I currently don't have the time to give things an in-depth look. As AbstractTrees looks pretty generic I suggest to include it in the package properly, not as extension. If you want, I invite you to co-maintain the STRTree package - then you can merge it yourself ;-)

[maxfreu]

Oh and tests would be cool, as always.