[Bug]: PyBaMM fails to import when user has already imported a library that uses mp.set_start_method

[aabills]

PyBaMM Version

develop

Python Version

3.11

Describe the bug

PyBaMM fails to import when importing a library that uses multiprocessing and has already set the start method to something other than fork, as a result of #3975. This should be apparent when reading the documentation for mp.set_start_method

Pending a long-term solution, I think that #3975 should at least have mp.set_start_method wrapped in a try-except -- most people, most of the time, do not really need the multiprocessing functionality, and I don't think pybamm should force those users to modify their multiprocessing configuration. It can throw a warning saying multiprocessing is not available or whatever.

Other solutions are welcome, of course.

Steps to Reproduce

module thing:

import multiprocessing as mp

mp.set_start_method("spawn")

def hello():
    print("hello, world")

script:

import thing
import pybamm

Relevant log output

❯ python main.py 
Traceback (most recent call last):
  File "/Users/abills/.../main.py", line 2, in <module>
    import pybamm
  File "/Users/abills/.../python3.11/site-packages/pybamm/__init__.py", line 204, in <module>
    from .solvers.base_solver import BaseSolver
  File "/Users/abills/.../python3.11/site-packages/pybamm/solvers/base_solver.py", line 18, in <module>
    mp.set_start_method("fork")
  File "/Users/abills/.../python3.11/multiprocessing/context.py", line 247, in set_start_method
    raise RuntimeError('context has already been set')
RuntimeError: context has already been set

[kratman]

@BradyPlanden Tagging you since the bug most likely comes from #3975