Extensible SR Storage SOP Class

Adds support for a new extensible IOD and SOP Class that allows new Content Item types to be used as they are added to the standard.

There is an increasing need for new Content Item types (such as SCOORD3D) and these are being added as needed to application-specific IODs and SOP Classes (such as the Colon CAD Supplement 126).

There is a need for a generic IOD and SOP Class to allow new applications to make use of these in a general way, yet this conflicts with the requirement to be able to render all content.