Interface for a potential that is artificially truncated, and thus which can
provide a zero-body potential that (approximately) corrects for the
truncation. The PotentialMaster gets a LRC potential from a potential
implementing this interface when the potential is given to the
PotentialMaster's setSpecies method. PotentialGroup also recognizes
this interface and adds a LRC potential to the PotentialMaster when a
PotentialTruncated is added to it via the type-specifying addPotential method.