CarpetX: Add function ArrayGroupSizeB in driver.cxx #815

Job	Run time
download-build-test (cpu, real32, debug)	13m 36s
download-build-test (cpu, real64, debug)	15m 8s
download-build-test (cpu, real64, optimize)	23m 19s
download-build-test (cuda, real32, debug)	28m 49s
download-build-test (cuda, real64, debug)	27m 20s
download-build-test (cuda, real64, optimize)	34m 49s
download-build-test (rocm, real64, debug)	20m 10s
download-build-test (rocm, real64, optimize)	26m 36s
	3h 9m 47s

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