prevent too many open files error in multiprocessing mode

[segasai]

With current redrock, when running rrdesi in multiprocessing mode (on my machine with python 3.12) I get this error:

$ env CUDA_VISIBLE_DEVICES='' RR_TEMPLATE_DIR=$HOME//tmp/rr_template_dir/ PYTHONPATH=py rrdesi  --outfile /tmp/aa.fits -i /home/skoposov/Downloads/coadd-main-dark-130.fits 
Running with 18 multiprocesing processes
WARNING:  using multiprocessing, but the OMP_NUM_THREADS
WARNING:  environment variable is not set- your system may
WARNING:  be oversubscribed.
Loading targets...
no archetypes or --archetypes-no-legendre; will turn off all the Legendre related arguments
Read and distribution of 1238 targets: 5.8 seconds
DEBUG: Reading templates from /home/skoposov//tmp/rr_template_dir/
Reading templates from ['/home/skoposov//tmp/rr_template_dir//rrtemplate-GALAXY-None-v2.6.fits', '/home/skoposov//tmp/rr_template_dir//rrtemplate-QSO-LOZ-v1.1.fits', '/home/skoposov//tmp/rr_template_dir//rrtemplate-QSO-HIZ-v1.1.fits', '/home/skoposov//tmp/rr_template_dir//rrtemplate-STAR-A-v0.1.fits', '/home/skoposov//tmp/rr_template_dir//rrtemplate-STAR-B-v0.1.fits', '/home/skoposov//tmp/rr_template_dir//rrtemplate-STAR-CV-v0.1.fits', '/home/skoposov//tmp/rr_template_dir//rrtemplate-STAR-F-v0.1.fits', '/home/skoposov//tmp/rr_template_dir//rrtemplate-STAR-G-v0.1.fits', '/home/skoposov//tmp/rr_template_dir//rrtemplate-STAR-K-v0.1.fits', '/home/skoposov//tmp/rr_template_dir//rrtemplate-STAR-M-v0.1.fits', '/home/skoposov//tmp/rr_template_dir//rrtemplate-STAR-WD-v0.1.fits']
INFO: rrtemplate-GALAXY-None-v2.6.fits GALAXY  PCA IGM=Calura12 z=-0.005-1.6997
...
INFO: rrtemplate-STAR-WD-v0.1.fits STAR WD PCA IGM=None z=-0.002-0.002
Read and broadcast of 11 templates: 0.0 seconds
Creating GPU context: 0.0 seconds
Rebinning templates: 6.3 seconds
Computing redshifts
  Scanning redshifts for template GALAXY
--- Process 0 raised an exception ---
Proc 0: Traceback (most recent call last):
Proc 0:   File "/home/skoposov/pyenv_dir/pyenv312/lib/python3.12/site-packages/redrock/external/desi.py", line 1087, in rrdesi
Proc 0:   File "/home/skoposov/pyenv_dir/pyenv312/lib/python3.12/site-packages/redrock/zfind.py", line 290, in zfind
Proc 0:   File "/home/skoposov/pyenv_dir/pyenv312/lib/python3.12/site-packages/redrock/zscan.py", line 1043, in calc_zchi2_targets
Proc 0:   File "/home/skoposov/pyenv_dir/pyenv312/lib/python3.12/site-packages/redrock/targets.py", line 379, in sharedmem_pack
Proc 0:   File "/home/skoposov/pyenv_dir/pyenv312/lib/python3.12/site-packages/redrock/targets.py", line 103, in sharedmem_pack
Proc 0:   File "/home/skoposov/pyenv_dir/pyenv312/lib/python3.12/site-packages/redrock/utils.py", line 180, in mp_array
Proc 0:   File "/usr/lib/python3.12/multiprocessing/context.py", line 130, in RawArray
Proc 0:   File "/usr/lib/python3.12/multiprocessing/sharedctypes.py", line 66, in RawArray
Proc 0:   File "/usr/lib/python3.12/multiprocessing/sharedctypes.py", line 41, in _new_value
Proc 0:   File "/usr/lib/python3.12/multiprocessing/heap.py", line 331, in __init__
Proc 0:   File "/usr/lib/python3.12/multiprocessing/heap.py", line 309, in malloc
Proc 0:   File "/usr/lib/python3.12/multiprocessing/heap.py", line 192, in _malloc
Proc 0:   File "/usr/lib/python3.12/multiprocessing/heap.py", line 166, in _new_arena
Proc 0:   File "/usr/lib/python3.12/multiprocessing/heap.py", line 89, in __init__
Proc 0: OSError: [Errno 24] Too many open files

Traceback (most recent call last):
  File "/home/skoposov/pyenv_dir/pyenv312/bin/rrdesi", line 10, in <module>
  File "/home/skoposov/pyenv_dir/pyenv312/lib/python3.12/site-packages/redrock/external/desi.py", line 1229, in rrdesi
  File "/home/skoposov/pyenv_dir/pyenv312/lib/python3.12/site-packages/redrock/external/desi.py", line 1087, in rrdesi
  File "/home/skoposov/pyenv_dir/pyenv312/lib/python3.12/site-packages/redrock/zfind.py", line 290, in zfind
  File "/home/skoposov/pyenv_dir/pyenv312/lib/python3.12/site-packages/redrock/zscan.py", line 1043, in calc_zchi2_targets
  File "/home/skoposov/pyenv_dir/pyenv312/lib/python3.12/site-packages/redrock/targets.py", line 379, in sharedmem_pack
  File "/home/skoposov/pyenv_dir/pyenv312/lib/python3.12/site-packages/redrock/targets.py", line 103, in sharedmem_pack
  File "/home/skoposov/pyenv_dir/pyenv312/lib/python3.12/site-packages/redrock/utils.py", line 180, in mp_array
  File "/usr/lib/python3.12/multiprocessing/context.py", line 130, in RawArray
  File "/usr/lib/python3.12/multiprocessing/sharedctypes.py", line 66, in RawArray
  File "/usr/lib/python3.12/multiprocessing/sharedctypes.py", line 41, in _new_value
  File "/usr/lib/python3.12/multiprocessing/heap.py", line 331, in __init__
  File "/usr/lib/python3.12/multiprocessing/heap.py", line 309, in malloc
  File "/usr/lib/python3.12/multiprocessing/heap.py", line 192, in _malloc
  File "/usr/lib/python3.12/multiprocessing/heap.py", line 166, in _new_arena
  File "/usr/lib/python3.12/multiprocessing/heap.py", line 89, in __init__
OSError: [Errno 24] Too many open files

The patch fixes it by explicitly join()'in the processes and closing one file handle.
( i haven't investigated, but i wouldn't be surprised if this issue occurs now due to recent python's switching to spawn from for fork)