Implement optional pickling for multicore samplers

[yannikschaelte]

This should enable using the multicore samplers on systems where multiprocessing pickles arguments (e.g. macos with python3.8).

Adds a basic pipeline test on MacOS systems, however the general test suite does not run (multiple errors with redis and processes ...). Could be extended, but should be fine for the moment, considering MacOS support tentative.

Fixes #286.

[codecov-commenter]

Codecov Report

Merging #320 into develop will decrease coverage by 0.02%.
The diff coverage is 96.15%.

@@             Coverage Diff             @@
##           develop     #320      +/-   ##
===========================================
- Coverage    83.39%   83.36%   -0.03%     
===========================================
  Files           87       87              
  Lines         4949     4972      +23     
===========================================
+ Hits          4127     4145      +18     
- Misses         822      827       +5     

Impacted Files	Coverage Δ
pyabc/platform_factory.py	90.90% <83.33%> (-9.10%)	⬇️
pyabc/sampler/multicore.py	98.18% <100.00%> (+0.22%)	⬆️
pyabc/sampler/multicore_evaluation_parallel.py	100.00% <100.00%> (ø)
pyabc/sampler/multicorebase.py	97.56% <100.00%> (+0.41%)	⬆️
pyabc/weighted_statistics.py	80.70% <0.00%> (-10.53%)	⬇️
pyabc/smc.py	96.16% <0.00%> (+0.29%)	⬆️
pyabc/sampler/redis_eps/cli.py	80.85% <0.00%> (+0.70%)	⬆️

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