RZ: Rename 1st Component to `r` #17849

Job	Run time
GCC 1D & 2D w/ MPI	16m 36s
GCC Minimal w/o MPI	29m 17s
GCC 3D & RZ w/ MPI, single precision	21m 4s
GCC ABLASTR w/o MPI	41s
save_pr_number	9s
Clang pywarpx	11m 18s
	1h 19m 5s

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