Improve DISF+DCSF summation #1192

Job	Run time
Build MDANSE and run tests on ubuntu-latest	16m 5s
Build MDANSE and run tests on ubuntu-latest	13m 48s
Build MDANSE and run tests on ubuntu-latest	13m 16s
Build MDANSE and run tests on ubuntu-latest	14m 34s
Build MDANSE and run tests on ubuntu-latest	14m 32s
Build MDANSE and run tests on windows-latest	16m 43s
Build MDANSE and run tests on windows-latest	17m 47s
Build MDANSE and run tests on windows-latest	17m 58s
Build MDANSE and run tests on windows-latest	17m 58s
Build MDANSE and run tests on windows-latest	17m 11s
Build MDANSE and run tests on macOS-latest	10m 50s
Build MDANSE and run tests on macOS-latest	8m 10s
Build MDANSE and run tests on macOS-latest	8m 17s
Build MDANSE and run tests on macOS-latest	9m 3s
Build MDANSE and run tests on macOS-latest	8m 41s
	3h 24m 53s

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