Minimal support for threads partitioning

[juntyr]

Fixes #106

This PR finally implements multithreading-based partitioning by limiting the scope:

• MPI partitioning is improved but its partitioning-before-partitioning is not fixed, just made more difficult to observe
• broadcasting args and folding results are kept minimal for now, both should in the future be serde-based
• threads partitioning requires an event log for now, in the future it could also support live event reporting with a bit of e.g. timsort
• no special resource sharing is done beyond exploiting sharing after Clone, e.g. by using Arcs inside scenario cogs
• no support for parallel pausing exists (see #68) and this is exploited to postpone some implementation decisions

Still, this is an important step forward!

TODOs:

• [x] implement result folding for MPI
• [x] check if final steps and time vote is still needed from partitioning
• [x] use more Arcs for large arrays in scenario cogs
  • [x] initial pass, includes Habitats
  • [x] DispersalSamplers are initialised too late, they would need to be created earlier (no more separate Scenario init and build steps)
• [x] one short refactoring pass
  • [x] fix event log recorder for MPI
  • [x] fix local partition lifetime API + MPI impl
• [x] check against MPI implementation for same results
• [x] check against previous MPI implementation for same results
• [x] check new in-memory dispersal sampler implementation
  • [x] fix with complete rewrite: when self-dispersal dominates, it may be distributed across several atoms

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1781 lines in your changes are missing coverage. Please review.

Project coverage is 15.59%. Comparing base (92d8ef1) to head (7ae8180).

Files	Patch %	Lines
necsim/partitioning/mpi/src/partition/common.rs	0.00%	203 Missing :warning:
necsim/partitioning/threads/src/lib.rs	0.00%	197 Missing :warning:
necsim/partitioning/threads/src/partition.rs	0.00%	194 Missing :warning:
...al_sampler/in_memory/packed_separable_alias/mod.rs	0.00%	137 Missing :warning:
necsim/impls/std/src/event_log/recorder.rs	0.00%	128 Missing :warning:
necsim/partitioning/threads/src/vote.rs	0.00%	105 Missing :warning:
...pler/in_memory/packed_separable_alias/dispersal.rs	0.00%	102 Missing :warning:
necsim/partitioning/mpi/src/lib.rs	0.00%	79 Missing :warning:
rustcoalescence/src/args/config/partitioning.rs	0.00%	45 Missing :warning:
necsim/partitioning/mpi/src/partition/root.rs	0.00%	38 Missing :warning:
... and 82 more

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #277 +/- ## ========================================== - Coverage 16.25% 15.59% -0.67% ========================================== Files 289 296 +7 Lines 20592 21469 +877 ========================================== Hits 3348 3348 - Misses 17244 18121 +877 ```

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[juntyr]

Madingley fg0size12 habitat with turnover map and 10^-3 sample percentage

==== speciation 0.1 (with seed 42) ====

new independent monolithic: 4192785 new independent threads: 4192785 new independent MPI: 4192785 new gillespie monolithic: 4192786 new skipping monolithic: 4192313

old skipping monolithic: 4193180 old gillespie monolithic: 4192921 old independent monolithic: 4192909 old independent MPI: 4192909

==== speciation 0.01 (all monolithic with entropy seeds) ==== old independent: 3995287, 3995788, 3995771, 3996757, 3996123 old skipping: 3995004, 3995460, 3995548, 3995990, 3994950, 3996074, 3995850, 3995151, 3996214, 3995426, 3996369, 3996164, 3995796, 3995925

new independent: 3996050, 3995425 new skipping: 3906695, 3905369, 3905860, 3905530, 3905530

[juntyr]

While MPI and threads clearly work, the separable dispersal sampler does NOT

[juntyr]

==== speciation 0.01 (all monolithic with entropy seeds) ====

new v2 independent: 3995604, 3995902, 3995285, 3996393, 3995210 new v2 skipping: 3995976, 3995564, 3996725, 3995754, 3995576, 3995929, 3995656, 3995466, 3995450, 3996128, 3995070, 3995100, 3995427, 3995644, 3996332, 3995801

[juntyr]

The new packed separable alias dispersal sampler should still be optimised further ... but it shall be good enough for now