MRG: refactor `calculate_gather_stats` to disallow repeated downsampling #5802

Job	Run time
Check	25s
coverage	10m 37s
minimum_rust_version	7m 8s
test (beta)	1m 32s
test (stable)	1m 34s
Lints (beta)	33s
test_all_features	14m 6s
Run tests under wasm32-wasi	1m 14s
test (windows)	4m 19s
Lints (stable)	28s
test (macos)	1m 37s
Check if wasm-pack builds a valid package for the sourmash crate	1m 21s
Check if cbindgen runs cleanly for generating the C headers	8m 46s
Publish (dry-run)	6m 19s
	59m 59s

Provide feedback