Fix calculation of CAv_Stokes diagnostic #439

Job	Run time
build-symmetric	2m 44s
build-asymmetric	6m 2s
build-repro	8m 20s
build-openmp	7m 53s
build-target	0s
test-layout	3m 12s
test-rotate	2m 26s
test-restart	3m 3s
test-nan	4m 30s
test-dim (t, time)	2m 7s
test-dim (l, horizontal length)	4m 9s
test-dim (q, enthalpy)	1m 42s
test-dim (r, density)	3m 23s
test-dim (h, vertical thickness)	2m 24s
test-dim (z, vertical coordinate)	1m 39s
test-regression	0s
test-grid	1m 42s
test-openmp	2m 29s
test-repro	1m 26s
cleanup-push	4s
cleanup-common	9s
cleanup-pr	0s
	59m 24s

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