LHfit : DVR based pixel selection + new ped variance extraction for MC

[gabemery]

Two elements implemented for (MC) processing :

• DVR based pixel selection for both data and MC before performing the likelihood fit. This is a small change to the pixels selected for data after volume reduction, but it allows to have the same selection in MC.
• A new method to extract the pedestal variance in MC : extract once at the start (like it is done for data using interleaved events). This is required to keep events where the full camera is selected and to avoid bias in the extraction in the case of dvr pixel selection

Draft to extend test coverage

[codecov]

Codecov Report

Attention: Patch coverage is 95.06173% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 72.92%. Comparing base (e0f6d78) to head (d2ca6ca).
Report is 4 commits behind head on main.

Files	Patch %	Lines
lstchain/reco/reconstructor.py	95.52%	3 Missing ⚠️
lstchain/reco/r0_to_dl1.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1245      +/-   ##
==========================================
+ Coverage   72.81%   72.92%   +0.11%     
==========================================
  Files         133      133              
  Lines       13911    13962      +51     
==========================================
+ Hits        10129    10182      +53     
+ Misses       3782     3780       -2     

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[chaimain]

Some simple changes based on code quality