Support for namedgraphs

[ymark]

Update the X3ML mappings definition file and the X3ML engine functionality so that particular information can be added under a particular namedgraph. The first apparent consequence is that the X3ML engine should output the results in a form that also outputs the value-URI of the namedgraph. An indicative format is TRIG. The main requirement is that we want to support the declaration of namedgraphs in many different levels:

• per mapping
• per domain
• per link
• entire mappings (all the mappings file) In the sequel we will use this issue as a master issue ticket for monitoring the activities.

[ymark]

A new branch has been created for developing the enhancements x3ml/namedgraphs