Add time panels
See
- https://gitlab.com/nfiesta/nfiesta_pg/-/wikis/Minimal-Working-Example#panels and
- https://gitlab.com/nfiesta/nfiesta_pg/-/wikis/Minimal-Working-Example#target-local-densities
- panels are defined for broad time window... better would be for one (or half) year
- data (t_target_data) are inserted only for some panels.