Define relations between different ALTO components

[cipriandinu]

Currently there is no option to express different relations between different ALTO objects (object = page, textblock, textline, string, illustration, composed block) except reading order and a default parent/child relation.

Might be usefull to be able to express other types of relations. Several examples:

"note" - a footnote textblock could be linked by its reference from text (a link between a string and a text block)
"key"/"value" relations - for specific documents for example invoices, "price" can be a "key" for the price itself, and price itself can be a "value" for "price" (here could be a link between two strings"
"relatedTo" - an illustration/table with its caption block
"..." - cells in a table linked to column/raw header
"parent"/"child" relations, other than default nested structure (page/textblocks/textlines/strings)

As technical implementation, we may use these as a separate collection of links (like "smLinks" in METS 1), a link being identified by a source, a type and retated object.