Add MultiFittingProblem class and example #1807

Job	Run time
style	12s
Coverage tests (macos-14 / Python 3.12)	1m 22s
Test examples (ubuntu-latest / Python 3.12)	20m 58s
Integration tests (ubuntu-latest / Python 3.12)	9m 37s
Unit tests (ubuntu-latest / Python 3.9)	1m 36s
Integration tests (windows-latest / Python 3.12)	11m 33s
Unit tests (ubuntu-latest / Python 3.10)	1m 40s
Integration tests (macos-13 / Python 3.12)	7m 55s
Unit tests (ubuntu-latest / Python 3.11)	1m 22s
Test examples (windows-latest / Python 3.12)	20m 58s
Integration tests (macos-14 / Python 3.12)	3m 24s
Unit tests (ubuntu-latest / Python 3.12)	1m 15s
Test examples (macos-13 / Python 3.12)	31m 33s
Unit tests (windows-latest / Python 3.9)	4m 32s
Test examples (macos-14 / Python 3.12)	8m 37s
Unit tests (windows-latest / Python 3.10)	2m 28s
Unit tests (windows-latest / Python 3.11)	4m 2s
Unit tests (windows-latest / Python 3.12)	3m 46s
Unit tests (macos-13 / Python 3.9)	1m 34s
Unit tests (macos-13 / Python 3.10)	1m 26s
Unit tests (macos-13 / Python 3.11)	2m 5s
Unit tests (macos-13 / Python 3.12)	2m 17s
Unit tests (macos-14 / Python 3.9)	1m 9s
Unit tests (macos-14 / Python 3.10)	1m 15s
Unit tests (macos-14 / Python 3.11)	1m 11s
	2h 27m 47s

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