Handle sparse likelihoods. #2339

Job	Run time
build (ubuntu-gcc-11-20.04)	17m 32s
build (ubuntu-gcc-11-22.04)	17m 31s
build (ubuntu-gcc-11)	17m 30s
build (ubuntu-gcc-12-debugoptimized)	17m 24s
build (ubuntu-gcc-13-DEBUG_MACHINE)	17m 24s
build (ubuntu-gcc-14-sanitize)	17m 25s
build (ubuntu-gcc-14)	17m 28s
build (ubuntu-clang-17)	17m 23s
build (macos-xcode-15-intel)	17m 43s
build (macos-xcode-16-arm)	17m 4s
build (windows)	17m 24s
	3h 11m 48s

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