SACM — A more generally available "citation" capability

A generally available "citation" capability is needed. This should be able to refer to points throughout the assurance case and outside. This is particularly required inside ArgumentDescriptions, Context, and narrative elements but potentially useful essentially everywhere throughout SACM. In addition to all the merits of referencing, it is potentially a means to avoid duplication.

How to do this from within an ArgumentDescription such as to localize within the referring element may require a different kind of mechanism than one that is associated with the element as a whole. Although not necessarily as critical, being able to cite a portion of an element is also important.

Add the following to attributes subsection of 11.1.2 Document, page 43:
citation:String The full citation of the document (bibliographical reference)