CarpetX: only multi-thread MFIter if AMReX supports it #681

Job	Run time
download-build-test (cpu, real32, debug)	12m 33s
download-build-test (cpu, real64, debug)	14m 49s
download-build-test (cpu, real64, optimize)	19m 6s
download-build-test (cuda, real32, debug)	29m 2s
download-build-test (cuda, real64, debug)	38m 16s
download-build-test (cuda, real64, optimize)	36m 57s
download-build-test (rocm, real64, debug)	21m 0s
download-build-test (rocm, real64, optimize)	20m 11s
	3h 11m 54s

Provide feedback