Generalize support for downsample diagnostics #13

Job	Run time
build-openmp	4m 33s
build-asymmetric	4m 47s
build-symmetric	4m 59s
build-repro	5m 20s
build-target	0s
test-dim (z, vertical coordinate)	2m 3s
test-openmp	2m 13s
test-dim (q, enthalpy)	2m 11s
test-grid	2m 19s
test-layout	1m 12s
test-restart	2m 33s
test-nan	2m 34s
test-dim (t, time)	2m 6s
test-rotate	2m 22s
test-dim (l, horizontal length)	2m 0s
test-dim (r, density)	1m 55s
test-dim (h, vertical thickness)	2m 15s
test-regression	0s
test-repro	1m 26s
cleanup-common	5s
cleanup-push	-1s
cleanup-pr	-1s
	46m 51s

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