Compute cleaning thresholds in lstchain_dvr_pixselector

[moralejo]

When used for determining the settings for the Data Volume Reduction (DVR) of a given run, the program now outputs also a .json file (adapted to the NSB conditions) for use in the lstchain_dl1ab step

[moralejo]

Also corrected a small error in earlier changes: in case more than one run was processed (in the "compute_dvr_settings" mode) the total number of (DL1) subruns processed was 10, instead of 10 per run, as intended.

[codecov]

Codecov Report

Attention: Patch coverage is 85.41667% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 72.93%. Comparing base (a453522) to head (68d0475).
Report is 6 commits behind head on main.

Files	Patch %	Lines
lstchain/scripts/lstchain_dvr_pixselector.py	85.41%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1239      +/-   ##
==========================================
+ Coverage   72.86%   72.93%   +0.07%     
==========================================
  Files         133      133              
  Lines       13819    13864      +45     
==========================================
+ Hits        10069    10112      +43     
- Misses       3750     3752       +2     

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