clean up NuclearDensityFunctor #1700

Job	Run time
macos-latest: clang++ Release Threads	35m 14s
macos-latest: clang++ Release OneTBB	4m 10s
macos-latest: clang++ Release PaRSEC	2m 35s
ubuntu-20.04: /usr/bin/g++-9 Release Threads	31m 52s
ubuntu-20.04: /usr/bin/g++-9 Release OneTBB	50s
ubuntu-20.04: /usr/bin/g++-9 Release PaRSEC	2m 14s
macos-latest: clang++ Debug Threads	54m 42s
macos-latest: clang++ Debug OneTBB	3m 30s
macos-latest: clang++ Debug PaRSEC	3m 56s
ubuntu-20.04: /usr/bin/g++-9 Debug Threads	41m 33s
ubuntu-20.04: /usr/bin/g++-9 Debug OneTBB	1m 39s
ubuntu-20.04: /usr/bin/g++-9 Debug PaRSEC	1m 36s
	3h 3m 51s

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