changed default alkalinity to prognostic in main setup only #463

Job	Run time
build-gotm-fabm-ersem-debian	3m 56s
build-fabm0d-gotm-ersem-debian	1m 21s
build-gotm-profilling	1m 47s
build-pyfabm-ersem-debian	1m 9s
	8m 13s

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