Reuse distto from statistics in polymer/diamond code, fix minor perfo…

[fweik]

…rmance issue, cleanup headers

[codecov]

Codecov Report

Merging #3402 into python will decrease coverage by <1%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           python   #3402   +/-   ##
======================================
- Coverage      86%     86%   -1%     
======================================
  Files         538     538           
  Lines       25232   25232           
======================================
- Hits        21761   21758    -3     
- Misses       3471    3474    +3

Impacted Files	Coverage Δ
...re/electrostatics_magnetostatics/p3m_send_mesh.cpp	100% <ø> (ø)	⬆️
...re/electrostatics_magnetostatics/p3m_send_mesh.hpp	100% <ø> (ø)	⬆️
src/core/electrostatics_magnetostatics/p3m.cpp	85% <0%> (-1%)	⬇️
src/core/particle_data.cpp	97% <0%> (-1%)	⬇️

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