fix merge_xray_quadtrees

[krzesi-mir]

Previously:

$ cargo run --bin merge_xray_quadtrees -- --output-directory ~/XRAYS/out_3/ ~/XRAYS/{1,2,3,4}
thread 'main' panicked at 'No parent root node found.', xray/src/bin/merge_xray_quadtrees.rs:140:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Now:

$ cargo run --bin merge_xray_quadtrees -- --output-directory ~/XRAYS/out_4/ ~/XRAYS/{1,2,3,4}
Building level 0: 1 / 1 [===================================================================================================================================================================================================] 100.00 % 0.91/s

[krzesi-mir]

Thanks Marco!