Make gaus_2d faster (2)

[AlexKurek]

before (but after #273)

    10935    0.200    0.000    0.204    0.000 /home/akurek/miniconda3/envs/pybdsf-313_local_temp/lib/python3.13/site-packages/bdsf/functions.py:202(gaus_2d)
    10935    0.205    0.000    0.208    0.000 /home/akurek/miniconda3/envs/pybdsf-313_local_temp/lib/python3.13/site-packages/bdsf/functions.py:202(gaus_2d)
    10935    0.222    0.000    0.226    0.000 /home/akurek/miniconda3/envs/pybdsf-313_local_temp/lib/python3.13/site-packages/bdsf/functions.py:202(gaus_2d)
    10935    0.196    0.000    0.200    0.000 /home/akurek/miniconda3/envs/pybdsf-313_local_temp/lib/python3.13/site-packages/bdsf/functions.py:202(gaus_2d)

after:

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
    10322    0.173    0.000    0.177    0.000 /home/akurek/miniconda3/envs/pybdsf-313_local_temp/lib/python3.13/site-packages/bdsf/functions.py:202(gaus_2d)
    10322    0.167    0.000    0.171    0.000 /home/akurek/miniconda3/envs/pybdsf-313_local_temp/lib/python3.13/site-packages/bdsf/functions.py:202(gaus_2d)
    10322    0.167    0.000    0.171    0.000 /home/akurek/miniconda3/envs/pybdsf-313_local_temp/lib/python3.13/site-packages/bdsf/functions.py:202(gaus_2d)
    10322    0.167    0.000    0.172    0.000 /home/akurek/miniconda3/envs/pybdsf-313_local_temp/lib/python3.13/site-packages/bdsf/functions.py:202(gaus_2d)
    10322    0.165    0.000    0.169    0.000 /home/akurek/miniconda3/envs/pybdsf-313_local_temp/lib/python3.13/site-packages/bdsf/functions.py:202(gaus_2d)

~18,4% faster. A lot is done in place now. I think this is now as fast as it gets.
There are very rare differences in the .gaul file at 13th decimal place, but this is normal.

[AlexKurek]

After 291f1e0 it gets faster by another ~24% :
10322 0.128 0.000 0.153 0.000 /home/akurek/miniconda3/envs/pybdsf-313_local_temp/lib/python3.13/site-packages/bdsf/functions.py:202(gaus_2d)

The .gaul file is OK.