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: https://github.com/neuronsimulator/nrn/issues/2842#issuecomment-2099841365

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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