WIP: ELC and MMM2D code duplication

[reinaual]

Fixes #2279

[codecov]

Codecov Report

Merging #2725 into python will decrease coverage by <1%.
The diff coverage is 96%.

@@           Coverage Diff            @@
##           python   #2725     +/-   ##
========================================
- Coverage      79%     79%     -1%     
========================================
  Files         508     491     -17     
  Lines       27049   25272   -1777     
========================================
- Hits        21621   20193   -1428     
+ Misses       5428    5079    -349

Impacted Files	Coverage Δ
...electrostatics_magnetostatics/elc_mmm2d_common.hpp	100% <100%> (ø)
src/core/electrostatics_magnetostatics/mmm2d.cpp	89% <100%> (-1%)	⬇️
src/core/electrostatics_magnetostatics/elc.cpp	70% <89%> (-1%)	⬇️
...al_sites/lb_inertialess_tracers_cuda_interface.cpp	0% <0%> (-100%)	⬇️
...rialization/ibm_cuda_particle_velocities_input.hpp	0% <0%> (-100%)	⬇️
src/core/actor/ActorList.cpp	0% <0%> (-80%)	⬇️
src/core/EspressoSystemInterface.cpp	40% <0%> (-52%)	⬇️
...tatics_magnetostatics/magnetic_non_p3m_methods.cpp	0% <0%> (-39%)	⬇️
src/core/PartCfg.hpp	80% <0%> (-20%)	⬇️
...rc/core/grid_based_algorithms/lb_interpolation.cpp	77% <0%> (-12%)	⬇️
... and 169 more

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[codecov]

Codecov Report

Merging #2725 into python will increase coverage by <1%.
The diff coverage is 96%.

@@           Coverage Diff           @@
##           python   #2725    +/-   ##
=======================================
+ Coverage      79%     79%   +<1%     
=======================================
  Files         508     509     +1     
  Lines       27049   26883   -166     
=======================================
- Hits        21621   21499   -122     
+ Misses       5428    5384    -44

Impacted Files	Coverage Δ
...electrostatics_magnetostatics/elc_mmm2d_common.hpp	100% <100%> (ø)
src/core/electrostatics_magnetostatics/mmm2d.cpp	89% <100%> (-1%)	⬇️
src/core/electrostatics_magnetostatics/elc.cpp	70% <89%> (-1%)	⬇️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[jngrad]

Is it ready for review?

[reinaual]

Yes, but @fweik wanted to wait until #2685 is finished before making any changes to the code.