Speed up batch-then-sample downsampling by returning 0-based indices #3631

Job	Run time
C++ Test Coverage (gets coverage of main branch, currently not main branch because no C++ on main)	8m 16s
Mypy Type Checks	6m 52s
Build + Test (C++) (Release, gcc, g++, 11)	8m 15s
format	10s
Build + Test (C++) (Release, clang, clang++, 14)	8m 21s
Pre-commit Checks	27s
pylint	4m 44s
Build + Test (C++) (Release, clang, clang++, 17, true)	8m 16s
Build + Test (C++) (Debug, gcc, g++, 11)	8m 17s
tidy	8m 15s
unittests	8m 18s
Build + Test (C++) (Debug, gcc, g++, 12)	8m 22s
Build + Test (C++) (Debug, clang, clang++, 14)	8m 19s
Build + Test (C++) (Debug, clang, clang++, 17, true)	8m 16s
Build + Test (C++) (clang, clang++, 14, Tsan)	8m 15s
Build + Test (C++) (clang, clang++, 14, Asan)	8m 20s
integrationtests-debug	1s
integrationtests-asan	1s
Comment Coverage Results	1s
dockerized-unittests	1s
integrationtests-release	1s
integrationtests-tsan	1s
	1h 51m 49s

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