Allow X splitting with FCI #4131

Job	Run time
CUDA	12m 56s
Fedora	19m 19s
CMake, PETSc unreleased, ADIOS2	26m 7s
Default options, Ubuntu 20.04	17m 1s
Optimised, static	22m 9s
Debug, shared	25m 51s
Shared, release, Ubuntu 20.04	18m 2s
Shared, OpenMP, 3D metrics	15m 0s
CMake, new PETSc	19m 1s
Coverage	13m 58s
	3h 9m 24s

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