Development, implementation, evaluation and scaling-up of physical activity referral schemes in Germany: protocol for a study using a co-production approach - Weissenfels, Anja; Geidl, Wolfgang; Mino, Eriselda; Naber, Inga; Klamroth, Sarah; Gelius, Peter; Abu-Omar, Karim; Pfeifer, Klaus (Friedrich-Alexander-Universität Erlangen-Nürnberg, 2021)