Derivs: Increase imax by the stencil radius in mask for derivatives #794

Job	Run time
download-build-test (cpu, real32, debug)	14m 41s
download-build-test (cpu, real64, debug)	15m 51s
download-build-test (cpu, real64, optimize)	24m 49s
download-build-test (cuda, real32, debug)	38m 50s
download-build-test (cuda, real64, debug)	34m 7s
download-build-test (cuda, real64, optimize)	27m 53s
download-build-test (rocm, real64, debug)	20m 13s
download-build-test (rocm, real64, optimize)	29m 23s
	3h 25m 47s

Provide feedback