WIP: Local particles

[fweik]

Description of changes:

• Replace the sparse array that was used for the local particle
  index by a hash map.
• Adding particles when initializing a new cell system is now handled
  by the regular particle sorting code instead of duplicated code.

This finally makes Espresso linear in memory consumption.
If the performance with the std::unordered_map is too bad
we could replace it by a better one e.g. from abseil, but for
the reduced complexity in bookkeeping I really would like to
get this in.

[codecov]

Codecov Report

Merging #2400 into python will decrease coverage by 1%.
The diff coverage is 90%.

@@           Coverage Diff           @@
##           python   #2400    +/-   ##
=======================================
- Coverage      77%     75%    -2%     
=======================================
  Files         498     494     -4     
  Lines       27891   27956    +65     
=======================================
- Hits        21492   21174   -318     
- Misses       6399    6782   +383

Impacted Files	Coverage Δ
src/core/domain_decomposition.hpp	100% <ø> (ø)	⬆️
src/core/communication.hpp	100% <ø> (ø)	⬆️
src/core/cells.hpp	100% <ø> (ø)	⬆️
src/core/ParticleCache.hpp	100% <ø> (ø)	⬆️
src/core/io/mpiio/mpiio.cpp	0% <0%> (ø)	⬆️
src/core/virtual_sites.cpp	87% <100%> (ø)	⬆️
src/core/collision.cpp	79% <100%> (-1%)	⬇️
src/core/nsquare.cpp	99% <100%> (-1%)	⬇️
src/core/PartCfg.hpp	100% <100%> (ø)	⬆️
src/core/ghosts.cpp	87% <100%> (-1%)	⬇️
... and 67 more

---

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[RudolfWeeber]

• for (auto c : local_cells) {
• for (int i = 0; i < c->n; i++) {

•  auto const &p = c->part[i];
  

•  check_index_ghost_part(p);
  

  }
  }
  }
  Should that loop go over ghost cells?

[RudolfWeeber]

Performance decreases by some 5% for a 1000 particle lj fluid at 0.1 volume fraction.
For 200 polymers of length 5 at 0.1 volume fraction, the slow down is some 15%.

[espresso-ci]

Your pull request does not meet our code formatting rules. To fix this, please do one of the following:

• You can download a patch with my suggested changes here, inspect it and make changes manually.
• You can directly apply it to your repository by running curl https://gitlab.icp.uni-stuttgart.de/espressomd/espresso/-/jobs/85710/artifacts/raw/style.patch | git apply -.
• You can run maintainer/CI/fix_style.sh to automatically fix your coding style. This is the same command that I have executed to generate the patch above, but it requires certain tools to be installed on your computer.

You can run gitlab-runner exec docker style afterwards to check if your changes worked out properly.

Please note that there are often multiple ways to correctly format code. As I am just a robot, I sometimes fail to identify the most aesthetically pleasing way. So please look over my suggested changes and adapt them where the style does not make sense.