Generic gas-filled scatterer (GAS) object/class.

A S4 class that provides slots to contain relevant metadata for gas-bearing scatterers belonging to the GAS-class. This object can include simple gas-filled bubbles to other scatterers with gas occlusions, swimbladders, and other internal features, if applicable. The default behavior for this type of object is to only reference the gaseous/fluid feature with excpetions that are model-dependent. See scatterer-class for a more detailed description on how this S4 object is organized.