Investigate negative flux errors

[juanep97]

Sometimes a negative flux is saved in the database as aperphot result. This gives an error downstream as in:

2023-10-10 20:42:11 - iop4lib.telescopes.cahat220 [cahat220.py:343] - DEBUG - Computing relative polarimetry for <AstroSource S4 0954+65 4>.                                                                2023-10-10 20:42:11 - iop4lib.telescopes.cahat220 [cahat220.py:417] - WARNING - polarimetry_group=[ReducedFit.objects.get(id=52269), ReducedFit.objects.get(id=52272), ReducedFit.objects.get(id=52271), ReducedFit.objects.get(id=52270)]: negative flux mean encountered while relative polarimetry for astrosource=AstroSource.objects.get(name='S4 0954+65 4') ??!! It will be nan, but maybe we should look into th
is...                                                                                                                                                                                                       
Traceback (most recent call last):                                                                                                                                                                          
  File "/home/vhega/miniconda3/envs/iop4/bin/iop4", line 8, in <module>                                                                                                                                     
    sys.exit(main())                                                                                                                                                                                        
  File "/home/vhega/iop4/iop4lib/iop4.py", line 195, in main                                                                                                                                                
    process_epochs(epochs_to_process, args.force_rebuild, check_remote_list=~args.skip_remote_file_list)                                                                                                    
  File "/home/vhega/iop4/iop4lib/iop4.py", line 65, in process_epochs                                                                                                                                       
    epoch.compute_relative_polarimetry()                                                                                                                                                                    
  File "/home/vhega/iop4/iop4lib/db/epoch.py", line 674, in compute_relative_polarimetry                                                                                                                    
    return list(map(f, clusters_L))                                                                                                                                                                         
  File "/home/vhega/iop4/iop4lib/db/epoch.py", line 672, in <lambda>                                                                                                                                        
    f = lambda x: Telescope.by_name(self.telescope).compute_relative_polarimetry(x, *args, **kwargs)                                                                                                        
  File "/home/vhega/iop4/iop4lib/telescopes/cahat220.py", line 419, in compute_relative_polarimetry                                                                                                         
    mag_inst = -2.5 * math.log10(flux_mean)                                                                                                                                                                 
ValueError: math domain error                                                                                                                                                                               
(iop4) vhega@quasar:~$     

Also, this error is not catched anywhere and breaks the pipeline execution.

Ocurrences (IOP4 version main @ d9d65f2).

• During relative photo-polarimetry reduction for source "S4 0954+65 4" in CAHA-T220/2023-10-09/caf-20231010-03:36:55-sci-agui.fit.

[morcuended]

Let's write here whatever instances of this kind of errors we find and debug, so we can make an informed decision about how to treat them.