Derivs: Increase imax by the stencil radius in mask for derivatives #772

Job	Run time
download-build-test (cpu, real32, debug)	12m 32s
download-build-test (cpu, real64, debug)	12m 23s
download-build-test (cpu, real64, optimize)	18m 51s
download-build-test (cuda, real32, debug)	38m 17s
download-build-test (cuda, real64, debug)	29m 2s
download-build-test (cuda, real64, optimize)	33m 40s
download-build-test (rocm, real64, debug)	21m 26s
download-build-test (rocm, real64, optimize)	22m 13s
	3h 8m 24s

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