added tiled computation of multiplications in XC #1882

Job	Run time
macos-latest: clang++ Release Threads	14m 54s
macos-latest: clang++ Release OneTBB	2m 15s
macos-latest: clang++ Release PaRSEC	2m 10s
ubuntu-20.04: /usr/bin/g++-9 Release Threads	27m 7s
ubuntu-20.04: /usr/bin/g++-9 Release OneTBB	1m 51s
ubuntu-20.04: /usr/bin/g++-9 Release PaRSEC	2m 13s
macos-latest: clang++ Debug Threads	5m 31s
macos-latest: clang++ Debug OneTBB	2m 5s
macos-latest: clang++ Debug PaRSEC	2m 30s
ubuntu-20.04: /usr/bin/g++-9 Debug Threads	14m 11s
ubuntu-20.04: /usr/bin/g++-9 Debug OneTBB	2m 1s
ubuntu-20.04: /usr/bin/g++-9 Debug PaRSEC	1m 44s
	1h 18m 32s

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