Add gain_selection flag for flatfield and pedestal events

[Hckjs]

closes #186

[codecov]

Codecov Report

Attention: Patch coverage is 97.08738% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 90.75%. Comparing base (1285c6a) to head (29a40e8).

Files	Patch %	Lines
src/ctapipe_io_lst/calibration.py	91.66%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #211      +/-   ##
==========================================
+ Coverage   90.67%   90.75%   +0.08%     
==========================================
  Files          22       22              
  Lines        2498     2520      +22     
==========================================
+ Hits         2265     2287      +22     
  Misses        233      233              

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only looked at the last commit