First semantics for signs

linked to #250

Since we want to add semantic on signs, we need to define a model for them.

We need to have tags for:

• indicating that there is a sign
• indication which model found this sign (and we need the version of this model, in the same tag or in another)
• indicating which precise sign it is (and since this classification can be done by another model, it would be nice to also add which model/version did the classification).