Fixes to Processes Manager on MacOS

[jeremyleung521]

Issue Number. Is this pull request related to any outstanding issues? If so, list the issue number.
#114

Describe the changes made. A clear and concise description of what the problem is and what you did to fix it. E.g. [...] was happening and I've changed [...] to fix it.
Since Python >= 3.8, multiprocessing defaults to spawn on MacOS (link). With spawn, new processes no longer share memory with their parents, leading to cases like in #114 when WESTPA is used with the processes work manager.

The downside to swapping back to the fork method (which is the default on other Unix systems) is that might crash the whole subprocess. Currently, we suggest users to install python 3.7 on MacOS, which defaults to fork and puts us at the same risk anyways.

Added a few lines to swap to fork when it's on a mac since the alternative is either: 1) Rewrite processes so it runs well with spawn or 2) Not working. 1) seemed non-trivial and I think swapping back to fork is a nice compromise.

Goals and Outstanding Issues. A clear and concise list of goals (to be) accomplished.

• Make Processes work manager work on MacOS + Python >=3.8

Major files changed.

• src/westpa/work_managers/processes.py

Status.

• Bug fix (non-breaking change which fixes an issue)
• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• All new and existing tests passed.

Additional context. Add any other context or screenshots about the pull request here.

[codecov-commenter]

Codecov Report

Merging #251 (b79ab5f) into westpa-2.0-restruct (747daee) will decrease coverage by 0.01%.
The diff coverage is 33.33%.

@@                   Coverage Diff                   @@
##           westpa-2.0-restruct     #251      +/-   ##
=======================================================
- Coverage                42.04%   42.02%   -0.02%     
=======================================================
  Files                      131      131              
  Lines                    16809    16834      +25     
=======================================================
+ Hits                      7067     7075       +8     
- Misses                    9742     9759      +17     

Flag	Coverage Δ
unittests	42.02% <33.33%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/westpa/work_managers/processes.py	91.80% <33.33%> (-1.31%)	⬇️
src/westpa/core/binning/binless_driver.py	18.75% <0.00%> (-0.61%)	⬇️
src/westpa/core/binning/mab.py	4.23% <0.00%> (-0.27%)	⬇️
src/westpa/core/data_manager.py	68.82% <0.00%> (ø)
src/westpa/work_managers/zeromq/work_manager.py	84.98% <0.00%> (+0.68%)	⬆️
src/westpa/core/binning/binless_manager.py	12.50% <0.00%> (+1.15%)	⬆️

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