**Acknowledgements**

RC wishes to acknowledge the invaluable help resulted from discussions with his coauthors during previous works; in particular, collaborations with Luca Barbui, Moritz Beyreuther, Corentin Caudron, Guillermo Cortés, Art Jolly, Philippe Lesage, Joachim Wassermann.

This review is partially based on the results of a previous project funded under the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Grant Agreement No. 749249 (VULCAN.ears).
