Better define "representation_hints" (in Metrics.json)

[lcodo]

Currently, representation_hints is now used to define some attributes to fine tune the visualization (e.j. optimization:minimal), and are associated to the metric concept.
Would be interesting detach these hints from the metrics concept, which should keep being an abstract concept (aka. operation). Could we associate it to Tool ?
And would be also interesting to model the attributes inside representation_hints.

[jmfernandez]

This has been moved to later, so we can retake it for data model 2.1