Add `sum_across_studies` to kda #1035

Job	Run time
Determine if CI should be skipped	6s
Unit tests with minimum dependency versions (ubuntu-latest, 3.8)	28m 6s
Corrector performance tests (ubuntu-latest, 3.8)	30m 39s
Unit tests (ubuntu-latest, 3.8)	35m 43s
Estimator performance tests (ubuntu-latest, 3.8)	7m 3s
Unit tests (ubuntu-latest, 3.9)	36m 9s
Unit tests (ubuntu-latest, 3.10)	37m 2s
CBMR ImportError tests (ubuntu-latest, 3.8)	1m 4s
Unit tests (ubuntu-latest, 3.11)	37m 17s
Smoke performance tests (ubuntu-latest, 3.8)	21m 39s
Unit tests (macos-latest, 3.8)	42m 56s
Unit tests (macos-latest, 3.9)	41m 56s
Unit tests (macos-latest, 3.10)	38m 13s
Unit tests (macos-latest, 3.11)	42m 57s
Upload coverage	1s
	6h 40m 51s

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