Fix CTAMARS-like energy estimation

[HealthyPear]

Closes #148

NOTE: the method getRMS in ROOT actually performs the standard deviation (see e.g. https://root.cern.ch/root/html534/TH1.html#TH1:GetRMS)

[codecov]

Codecov Report

Merging #156 (6f89fb3) into master (e1ce3e6) will increase coverage by 0.11%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
+ Coverage   66.14%   66.25%   +0.11%     
==========================================
  Files          24       24              
  Lines        2283     2291       +8     
==========================================
+ Hits         1510     1518       +8     
  Misses        773      773              

Impacted Files	Coverage Δ
protopipe/scripts/data_training.py	93.21% <85.71%> (+0.12%)	⬆️
protopipe/scripts/write_dl2.py	82.25% <85.71%> (+0.23%)	⬆️

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