Pvcode + Cvode improvements #4147

Job	Run time
CMake, PETSc unreleased, ADIOS2	25m 48s
Default options, Ubuntu 20.04	19m 1s
Optimised, static	21m 38s
Debug, shared	27m 13s
Shared, release, Ubuntu 20.04	17m 57s
Shared, OpenMP, 3D metrics	14m 6s
CMake, new PETSc	18m 55s
Coverage	14m 25s
CUDA	12m 34s
Fedora	18m 38s
	3h 10m 15s

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