Add 'is_impossible' tag to annotated datasets

[Rob192]

Problem description

We are using knowledge base formated per the squad v2 format, see https://etalab-ia.github.io/knowledge-base/piaf/howtos/integrate.html
When using data from Piaf Anno, the data is formated in v1 format

Solution description

Add is impossible keys to dataset generated with piaf anno

[guillim]

Good point ! Anyone could seize this issue : it's a good "first-commit" on Piaf