Make check_islands_for_overlap faster

[AlexKurek]

It makes the entire run faster by ~2x. I think the greater the size of the image, the greater the speed increase.

For 1.5 GB FITS, 19845 x 19845 pixels:
Out of 25,889 lines in the .gaul file, there are 4 that have numerical differences at the 6th decimal place.

before:

PyBDSF running time: 0:41:0
PyBDSF running time: 0:38:12
   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        4 1005.956  251.489 1019.746  254.936 /home/akurek/miniconda3/envs/pybdsf-313_local_temp/lib/python3.13/site-packages/bdsf/wavelet_[atrous.py:681](http://atrous.py:681/)(check_islands_for_overlap)

after:

PyBDSF running time: 0:22:28
PyBDSF running time: 0:20:54
        4    3.252    0.813   15.651    3.913 /home/akurek/miniconda3/envs/pybdsf-313_local_temp/lib/python3.13/site-packages/bdsf/wavelet_[atrous.py:681](http://atrous.py:681/)(check_islands_for_overlap)
        4    3.020    0.755   15.355    3.839 /home/akurek/miniconda3/envs/pybdsf-313_local_temp/lib/python3.13/site-packages/bdsf/wavelet_[atrous.py:681](http://atrous.py:681/)(check_islands_for_overlap)

PyBDSF options used: https://github.com/user-attachments/files/27450974/run_pybdsf.py.