further restrict IsolatedProcess by setting multiprocessing start method

[anilbey]

This is to ensure all workers of IsolatedProcesss are going to be started via the fork mode, not the spawn mode.
Note: fork mode passes the parameters from the parent to child. Spawn does not (it leads to unexpected results when used with NEURON).
Source: neuronsimulator/nrn#2842 (comment)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.24%. Comparing base (fcf8ad0) to head (fe3b446).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #181   +/-   ##
=======================================
  Coverage   91.23%   91.24%           
=======================================
  Files          95       95           
  Lines        5626     5628    +2     
=======================================
+ Hits         5133     5135    +2     
  Misses        493      493           

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