Generalize support for downsample diagnostics #10

Job	Run time
build-symmetric	5m 3s
build-openmp	3m 37s
build-asymmetric	3m 44s
build-repro	7m 19s
build-target	0s
test-rotate	2m 8s
test-layout	52s
test-nan	1m 50s
test-restart	1m 41s
test-grid	2m 8s
test-openmp	2m 33s
test-dim (z, vertical coordinate)	2m 36s
test-dim (q, enthalpy)	2m 4s
test-dim (t, time)	1m 57s
test-dim (l, horizontal length)	1m 43s
test-dim (h, vertical thickness)	2m 7s
test-dim (r, density)	2m 4s
test-regression	0s
test-repro	1m 38s
cleanup-common	7s
cleanup-push	0s
cleanup-pr	0s
	45m 11s

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