Updated all the examples' YAML files to use the new periodic flag.

examples/SineWavePotential_1D/makeIC.py

examples/SineWavePotential_1D/sineWavePotential.yml

examples/SineWavePotential_2D/makeIC.py

examples/SineWavePotential_2D/sineWavePotential.yml

examples/SineWavePotential_3D/makeIC.py

examples/SineWavePotential_3D/sineWavePotential.yml

examples/SmallCosmoVolume/small_cosmo_volume.yml

examples/SmallCosmoVolume_DM/small_cosmo_volume_dm.yml

examples/SmallCosmoVolume_cooling/small_cosmo_volume.yml

examples/SmoothedMetallicity/makeIC.py

examples/SmoothedMetallicity/smoothed_metallicity.yml

examples/SodShockSpherical_2D/makeIC.py

examples/SodShockSpherical_2D/sodShock.yml

examples/SodShockSpherical_3D/makeIC.py

examples/SodShockSpherical_3D/sodShock.yml

examples/SodShock_1D/makeIC.py

examples/SodShock_1D/sodShock.yml

examples/SodShock_2D/makeIC.py

examples/SodShock_2D/sodShock.yml

examples/SodShock_3D/makeIC.py