Derivs: Increase imax by the stencil radius in mask for derivatives #774

Job	Run time
download-build-test (cpu, real32, debug)	12m 2s
download-build-test (cpu, real64, debug)	12m 33s
download-build-test (cpu, real64, optimize)	23m 9s
download-build-test (cuda, real32, debug)	28m 39s
download-build-test (cuda, real64, debug)	29m 53s
download-build-test (cuda, real64, optimize)	27m 40s
download-build-test (rocm, real64, debug)	24m 57s
download-build-test (rocm, real64, optimize)	20m 12s
	2h 59m 5s

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