[MCC] Improve the Petri nets repository and characteristics

[saucisson]

The Petri nets repository is a repository of models in Petri nets, that come together with characteristics. Its API should be improved.

The repository API could allow users to upload analysis results, such as the values for characteristics, for instance computed from a dedicated computer (BlueWhale).

New characteristics should be added and computed.

[saucisson]

Hooks should trigger computations when a new model is added or updated, or when a characteristic is added or updated.

The hook could contain the docker image name of the tool to run to compute the characteristic.