Enable non-const iteration over `Options` #5086

Job	Run time
Fedora	18m 11s
CUDA	12m 16s
Shared, release	15m 50s
CMake, new PETSc	14m 48s
Default options, Ubuntu 22.04	14m 25s
Optimised, static	16m 24s
Debug, shared	26m 2s
CMake, PETSc unreleased, ADIOS2	24m 6s
Shared, OpenMP, 3D metrics	20m 23s
	2h 42m 25s

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