Define anyURI explicitly for internal references

[luciansmith]

By design and by convention, attributes of type 'anyURI' (particularly the 'source' attribute of a Model) may reference SED-ML elements by using a hashtag before the referenced ID to distinguish them from URLs or relative pathnames.

However, we don't actually say this anywhere in the spec; we just use it that way (cf listing 2.27 in section 2.2.4). We need to be explicit about this and instead of describing the anyURI data type as having two common use cases (URL and relative filename) we need to say there are three (URL, relative filename, SED-ML id).

Alternatively, we could in theory allow no-hashtag strings as references to any SED-ML ids that match, in preference to relative filenames that match. I would not recommend this, however.