clean up NuclearDensityFunctor #1933

Job	Run time
macos-latest: clang++ Release Threads	14m 47s
macos-latest: clang++ Release OneTBB	1m 52s
macos-latest: clang++ Release PaRSEC	2m 38s
ubuntu-20.04: /usr/bin/g++-9 Release Threads	24m 0s
ubuntu-20.04: /usr/bin/g++-9 Release OneTBB	1m 16s
ubuntu-20.04: /usr/bin/g++-9 Release PaRSEC	1m 29s
macos-latest: clang++ Debug Threads	19m 6s
macos-latest: clang++ Debug OneTBB	1m 48s
macos-latest: clang++ Debug PaRSEC	1m 49s
ubuntu-20.04: /usr/bin/g++-9 Debug Threads	47m 46s
ubuntu-20.04: /usr/bin/g++-9 Debug OneTBB	1m 55s
ubuntu-20.04: /usr/bin/g++-9 Debug PaRSEC	2m 0s
	2h 0m 26s

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