Minor refactoring of refined injection for AddPlasmaFlux #20548

Job	Run time
GCC 1D & 2D w/ MPI, QED tools	27m 35s
GCC 3D & RZ w/ MPI, single precision	34m 34s
GCC Minimal w/o MPI	16m 0s
GCC ABLASTR w/o MPI	39s
Clang pywarpx	7m 21s
	1h 26m 9s

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