Add MultiFittingProblem class and example #1871

Job	Run time
style	13s
Coverage tests (macos-14 / Python 3.12)	7m 18s
Integration tests (ubuntu-latest / Python 3.12)	10m 17s
Test examples (ubuntu-latest / Python 3.12)	20m 54s
Integration tests (windows-latest / Python 3.12)	11m 13s
Unit tests (ubuntu-latest / Python 3.9)	1m 14s
Test examples (windows-latest / Python 3.12)	19m 39s
Integration tests (macos-13 / Python 3.12)	11m 46s
Unit tests (ubuntu-latest / Python 3.10)	1m 21s
Test examples (macos-13 / Python 3.12)	34m 25s
Integration tests (macos-14 / Python 3.12)	3m 57s
Unit tests (ubuntu-latest / Python 3.11)	1m 9s
Test examples (macos-14 / Python 3.12)	8m 1s
Unit tests (ubuntu-latest / Python 3.12)	1m 24s
Unit tests (windows-latest / Python 3.9)	2m 44s
Unit tests (windows-latest / Python 3.10)	2m 27s
Unit tests (windows-latest / Python 3.11)	4m 41s
Unit tests (windows-latest / Python 3.12)	3m 50s
Unit tests (macos-13 / Python 3.9)	2m 59s
Unit tests (macos-13 / Python 3.10)	1m 43s
Unit tests (macos-13 / Python 3.11)	3m 27s
Unit tests (macos-13 / Python 3.12)	1m 28s
Unit tests (macos-14 / Python 3.9)	1m 1s
Unit tests (macos-14 / Python 3.10)	1m 18s
Unit tests (macos-14 / Python 3.11)	50s
	2h 39m 19s

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