Implement local meta-index to do source-selection

[lecoqlibre]

Each instance could have a meta-index that would indicate if we can find users with some skills or working in some city.

We could have something like:

<> a ex:MetaIndex.

<#skill12> a ex:MetaIndexEntry;
  ex:forSkill 12;
  ex:indexDocument <link/to/the/index/document>.

This would tell us if we can find users with (skill12 OR skill500 OR working at toulouse) but this won't tell if we can find users with (skill12 AND skill500 AND working at toulouse) @pchampin.

Possible alternatives:

• Think about having a querying agent on each instance #24
• Study the TREE hypermedia strategy #27