Slightly adjust estimate_rcut to improve PBC integral accuracy #993

Job	Run time
macos-build (3.10)	46m 41s
linux-build-aarch64 (ubuntu-20.04, cp39-cp39)	11m 35s
linux-build (ubuntu-20.04, 3.8)	1h 9m 37s
linux-build (ubuntu-20.04, 3.12)	1h 5m 49s
	3h 13m 42s

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