Address recent review to make carbon exchange configurable #4

Job	Run time
build-asymmetric	2m 40s
build-openmp	2m 35s
build-coverage	2m 41s
build-coupled-api	2m 44s
build-repro	3m 30s
build-opt	5m 27s
build-symmetric	2m 22s
check-style-and-docstrings	3m 5s
build-target	-1s
build-opt-target	-1s
test-layout	1m 39s
test-nan	1m 48s
test-dim (l, horizontal length)	1m 43s
test-dim (z, vertical coordinate)	1m 49s
test-restart	1m 43s
test-dim (q, enthalpy)	1m 52s
test-dim (t, time)	1m 57s
test-dim (r, density)	1m 47s
test-rotate	1m 50s
test-dim (h, vertical thickness)	1m 51s
test-regression	-2s
test-openmp	2m 4s
test-grid	1m 47s
run-coverage	1m 29s
test-repro	1m 31s
run-timings	1m 56s
compare-timings	-1s
cleanup-common	6s
cleanup-pr	-1s
cleanup-push	5s
	51m 55s

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