Truncation of Gaussian expansion of integral kernel in SeparatedConvo… #1978

Job	Run time
macos-latest: clang++ Release Threads	14m 45s
macos-latest: clang++ Release OneTBB	3m 32s
macos-latest: clang++ Release PaRSEC	3m 35s
ubuntu-20.04: /usr/bin/g++-9 Release Threads	36m 39s
ubuntu-20.04: /usr/bin/g++-9 Release OneTBB	2m 57s
ubuntu-20.04: /usr/bin/g++-9 Release PaRSEC	3m 0s
macos-latest: clang++ Debug Threads	22m 55s
macos-latest: clang++ Debug OneTBB	2m 42s
macos-latest: clang++ Debug PaRSEC	2m 48s
ubuntu-20.04: /usr/bin/g++-9 Debug Threads	47m 36s
ubuntu-20.04: /usr/bin/g++-9 Debug OneTBB	2m 29s
ubuntu-20.04: /usr/bin/g++-9 Debug PaRSEC	4m 2s
	2h 27m 0s

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