Generalize support for downsample diagnostics #14

Job	Run time
build-target	4m 29s
build-repro	7m 54s
build-openmp	5m 35s
build-asymmetric	4m 17s
build-symmetric	5m 21s
test-nan	2m 44s
test-grid	1m 57s
test-layout	1m 21s
test-regression	2m 10s
test-restart	1m 51s
test-dim (z, vertical coordinate)	1m 59s
test-dim (t, time)	2m 7s
test-dim (l, horizontal length)	2m 28s
test-dim (q, enthalpy)	2m 9s
test-dim (h, vertical thickness)	2m 22s
test-rotate	2m 2s
test-dim (r, density)	2m 34s
test-openmp	1m 55s
test-repro	1m 41s
cleanup-common	8s
cleanup-pr	0s
cleanup-push	0s
	57m 4s

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