Describe the feature / issue
We require the ability to add artificial or "dummy" atoms to a Species for the purposes of being able to define interacting sites such as:

• Shells, permitting the use of core-shell models within Dissolve
• Soft energy "zones", permitting trial implementation of dynamic geometry coordination restraints etc.

Of key importance is that these additional atoms do not constitute parts of the correlation function calculations, for instance in the GR and SQ modules, and so must be treated / partitioned carefully.