Enable non-const iteration over `Options` #5087

Job	Run time
Fedora	19m 48s
CUDA	11m 36s
CMake, PETSc unreleased, ADIOS2	25m 29s
Default options, Ubuntu 22.04	14m 41s
Shared, release	17m 0s
Shared, OpenMP, 3D metrics	14m 59s
Debug, shared	27m 27s
CMake, new PETSc	14m 5s
Optimised, static	17m 0s
	2h 42m 5s

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