[ut] einsum_tot/ijk_mn_eq_ij_mn_times_kj_mn : how to compute ref_result #930

Job	Run time
macos-latest: clang++ Release Pthreads	35m 8s
macos-latest: clang++ Release PaRSEC	46m 17s
macos-latest: clang++ Debug Pthreads	39m 6s
macos-latest: clang++ Debug PaRSEC	19m 57s
macos-latest: /usr/local/bin/g++-10 Release Pthreads	35m 7s
macos-latest: /usr/local/bin/g++-10 Release PaRSEC	40m 11s
macos-latest: /usr/local/bin/g++-10 Debug Pthreads	36m 30s
macos-latest: /usr/local/bin/g++-10 Debug PaRSEC	29m 22s
	4h 41m 38s

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