Description of the unit system for snapshots is now optional

examples/CosmoVolume/cosmoVolume.yml

examples/EAGLE_12/eagle_12.yml

examples/EAGLE_25/eagle_25.yml

examples/EAGLE_50/eagle_50.yml

examples/ExternalPointMass/externalPointMass.yml

examples/MultiTypes/multiTypes.yml

examples/SedovBlast/sedov.yml

examples/SodShock/sodShock.yml

examples/UniformBox/uniformBox.yml

examples/parameter_example.yml