Spaql query - Define policy for optional properties

[ghost]

Some times we have to switch some patterns to OPTIONAL because if not, the query returns no result. It might be useful to define which property can be seen as optional (see for instance #21 for a pattern using the skos:notation predicate that needed to be turned to optional).

Plus, if we consider a pattern as OPTIONAL, we have to deal with the situation where it is not present in the ui.

More precisely:

• is skos:prefLabel mandatory for a classification ? For an item ? For a level ? For a correspondence ?
• the same with skos:notation.