added fnat - energy scatter plots

[mgiulini]

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Closes #720

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (31155b3) 70.25% compared to head (ee66e46) 70.25%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #725   +/-   ##
=======================================
  Coverage   70.25%   70.25%           
=======================================
  Files          78       78           
  Lines        6967     6967           
=======================================
  Hits         4895     4895           
  Misses       2072     2072           

Files	Coverage Δ
src/haddock/libs/libplots.py	96.75% <100.00%> (ø)

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