(updating) links to external collaborators

• dgallaun -> https://de.linkedin.com/in/dennis-gallaun-a31027243
• jmeichsner -> https://tu-dresden.de/bu/umwelt/geo/ipg/astro/weltraumastrometrie/mitarbeiter
• jgrossmann -> https://www.jp-g.de/
• ahaupt -> https://www.linkedin.com/in/a-haupt/
• pgupta -> https://www.fim.uni-passau.de/diskrete-mathematik/team/
• dreding -> https://stackexchange.com/users/5471468/damian-reding ?
• Simona Boyadzhiyska -> https://www.birmingham.ac.uk/schools/mathematics/people/navigation.aspx?ReferenceId=200529&Name=ms-simona-boyadzhiyska