Pass selected year to screenshotter for the PDF export
For the CSV export there is a long standing requirement to show entire
trajectories, and not just the selected river. We used to pick a
trajectory from the hoofdtrajecten list, which don't always overlap
100% with the rivers and led to confusion. Now we take the river and
expand it, if possible, at the front and end using the hoofdtrajecten
(see NamedReach.expanded_reaches()).