This schema regroups all the definitions pertaining to the beam definition at the start of the machine. Note that I defined the child elements such that they can occur in any order. In addition, I constrained the user to specify certain key parameters. It is safe to assume (I think) that most lattice codes will at least need to know what energy, charge and mass of particle to use. Tracking codes may need more but since this schema is supposed to cover all possible codes including pure matrix transport calculations, it is better not to force the user to specify these extra beam parameters. Should we also specify default values for these parameters that can be ommitted?