Elastic shelled scatterer (ESS) object/class.

A S4 class that provides slots to contain relevant metadata for elastic shelled scatterers/objects belonging to the ESS-class. This object can be created using values for both an outer shell and internal tissues, if applicable. The default behavior for this type of this object is to only reference the outer shell with few exceptions that are model-dependent. See scatterer-class for a more detailed description on how this S4 object is organized.