Improve DISF+DCSF summation #1195

Job	Run time
Build MDANSE and run tests on ubuntu-latest	16m 7s
Build MDANSE and run tests on ubuntu-latest	13m 25s
Build MDANSE and run tests on ubuntu-latest	13m 20s
Build MDANSE and run tests on ubuntu-latest	14m 25s
Build MDANSE and run tests on ubuntu-latest	14m 22s
Build MDANSE and run tests on windows-latest	17m 39s
Build MDANSE and run tests on windows-latest	17m 11s
Build MDANSE and run tests on windows-latest	17m 33s
Build MDANSE and run tests on windows-latest	17m 57s
Build MDANSE and run tests on windows-latest	17m 52s
Build MDANSE and run tests on macOS-latest	9m 26s
Build MDANSE and run tests on macOS-latest	9m 34s
Build MDANSE and run tests on macOS-latest	7m 18s
Build MDANSE and run tests on macOS-latest	6m 53s
Build MDANSE and run tests on macOS-latest	6m 47s
	3h 19m 49s

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