fix: make multi-processing in baseline models more consistent

[sibre28]

Closes #907

Set Context of ProcesspoolExecutors to spawn

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ PYTHON	black	2	0	0	1.07s
✅ PYTHON	mypy	2		0	3.01s
✅ PYTHON	ruff	2	0	0	0.4s
✅ REPOSITORY	git_diff	yes		no	0.67s

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.77%. Comparing base (d3f81dc) to head (7e40c7b).
Report is 31 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #909   +/-   ##
=======================================
  Coverage   97.77%   97.77%           
=======================================
  Files         122      122           
  Lines        6592     6594    +2     
=======================================
+ Hits         6445     6447    +2     
  Misses        147      147           

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[lars-reimann]

Doesn't fix the issue with polars 1.0.0, unfortunately, but still a needed change.

[lars-reimann]

🎉 This PR is included in version 0.27.0 🎉

The release is available on:

• v0.27.0
• GitHub release

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