RDF prerequisites check, energy definition and kinetic energy property

• For RDF method / property the structure must have pbc and cell: this should be statically checked, if structure available as literal, otherwise a domain-specific error message.
• Specify "energy" more precisely: explain in docs that property "energy" is the potential energy, i.e. full energy without kinetic energy of the nuclei.
• Kinetic energy: add something like kinetic_energy property (needs no calculator and the algorithm is trivial).