Add ability to the parser to treat optional parameters

examples/CosmoVolume/cosmoVolume.yml

examples/ExternalPointMass/externalPointMass.yml

examples/SedovBlast/sedov.yml

examples/SodShock/sodShock.yml

examples/UniformBox/uniformBox.yml

examples/parameter_example.yml

src/engine.c

src/parser.c

src/parser.h

src/space.c