[FIX] peakfit: limit multiprocessing to 2 processes during fitting to avoid crashes #735

Job	Run time
Dask / dask ubuntu-latest py3.10 qt5.15.*	6m 0s
Usual / build (macOS-latest, 3.10, orange-released, 5.15., false) / orange-released macOS-latest py3.10 qt5.15.	4m 40s
Usual / build (macOS-latest, 3.10, orange-released, 6.6., false) / orange-released macOS-latest py3.10 qt6.6.	1m 13s
Usual / build (macOS-latest, 3.11, orange-latest, 5.15., false) / orange-latest macOS-latest py3.11 qt5.15.	5m 27s
Usual / build (ubuntu-latest, 3.10, orange-released, 6.6., false) / orange-released ubuntu-latest py3.10 qt6.6.	1m 54s
Usual / build (macOS-latest, 3.9, orange-oldest, 5.15., false) / orange-oldest macOS-latest py3.9 qt5.15.	10m 44s
Usual / build (ubuntu-latest, 3.11, orange-latest, 5.15., false) / orange-latest ubuntu-latest py3.11 qt5.15.	6m 23s
Usual / build (ubuntu-latest, 3.10, orange-released, 5.15., false) / orange-released ubuntu-latest py3.10 qt5.15.	4m 50s
Usual / build (ubuntu-latest, 3.9, orange-oldest, 5.15., false) / orange-oldest ubuntu-latest py3.9 qt5.15.	4m 6s
Usual / build (windows-latest, 3.10, orange-released, 6.6., false) / orange-released windows-latest py3.10 qt6.6.	2m 29s
Usual / build (windows-latest, 3.10, orange-released, 5.15., false) / orange-released windows-latest py3.10 qt5.15.	6m 34s
Usual / build (windows-latest, 3.11, orange-latest, 5.15., false) / orange-latest windows-latest py3.11 qt5.15.	7m 37s
Usual / build (windows-latest, 3.9, orange-oldest, 5.15., false) / orange-oldest windows-latest py3.9 qt5.15.	5m 15s
	1h 7m 12s

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