Allow the injection_style to be the same for all sources #1259

Job	Run time
GCC 1D & 2D w/ MPI	34m 14s
GCC 3D & RZ w/ MPI, single precision	51m 27s
GCC Minimal w/o MPI	36m 0s
GCC ABLASTR w/o MPI	7m 23s
Clang pywarpx	23m 45s
	2h 32m 49s

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