`ThreadPool::default_nthread` is public and static #1555

Job	Run time
macos-latest: clang++ Release Threads	24m 58s
macos-latest: clang++ Release OneTBB	3m 34s
macos-latest: clang++ Release PaRSEC	3m 30s
ubuntu-20.04: /usr/bin/g++-9 Release Threads	49m 54s
ubuntu-20.04: /usr/bin/g++-9 Release OneTBB	2m 20s
ubuntu-20.04: /usr/bin/g++-9 Release PaRSEC	3m 39s
macos-latest: clang++ Debug Threads	38m 55s
macos-latest: clang++ Debug OneTBB	3m 54s
macos-latest: clang++ Debug PaRSEC	4m 41s
ubuntu-20.04: /usr/bin/g++-9 Debug Threads	1h 15m 2s
ubuntu-20.04: /usr/bin/g++-9 Debug OneTBB	2m 44s
ubuntu-20.04: /usr/bin/g++-9 Debug PaRSEC	1m 52s
	3h 35m 3s

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