Tests

Add overloaded getters for G1, G2, G3, that give the value at a parti… #542

Job	Run time
Fedora	18m 6s
CUDA	12m 36s
CMake, PETSc unreleased, ADIOS2	1m 38s
Default options, Ubuntu 20.04	1m 51s
Optimised, static	1m 51s
Debug, shared	1m 50s
Shared, release, Ubuntu 20.04	1m 50s
Shared, OpenMP, 3D metrics	1m 33s
CMake, new PETSc	1m 35s
	42m 50s

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