Merge branch 'master' into initialisation_fixes

CONTRIBUTING.md

examples/plot_tasks.py

examples/plot_tasks_MPI.py

src/task.c

src/task.h

src/timestep.h

tests/test125cells.c

tests/test125cells.sh.in

tests/test27cells.c

tests/test27cells.sh.in

tests/test27cellsPerturbed.sh.in

tests/testFFT.c

tests/testKernelGrav.c

tests/testMaths.c

tests/testPair.sh.in

tests/testPairPerturbed.sh.in

tests/testParser.sh.in

tests/tolerance_27_normal.dat

tests/tolerance_27_perturbed.dat

tests/tolerance_pair_normal.dat