Expose local_particles only thru function calls

[fweik]

• Access local_particles (local particle map) by functions instead of directly
• Removed unused code
• Simplified PartCfg

[codecov]

Codecov Report

Merging #3501 into python will decrease coverage by <1%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           python   #3501   +/-   ##
======================================
- Coverage      87%     87%   -1%     
======================================
  Files         534     534           
  Lines       24357   24357           
======================================
- Hits        21204   21203    -1     
- Misses       3153    3154    +1

Impacted Files	Coverage Δ
src/core/global.cpp	83% <ø> (ø)	⬆️
src/core/particle_data.hpp	100% <ø> (ø)	⬆️
src/core/particle_data.cpp	96% <ø> (-1%)	⬇️
src/core/electrostatics_magnetostatics/p3m.cpp	85% <0%> (-1%)	⬇️
src/core/polymer.cpp	98% <0%> (+5%)	⬆️

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[jngrad]

LGTM, pending CI

[fweik]

Unfortunately now clang-tidy can analyze the code ^^