refactor: simplified dayCellFactory for date selector

Instead of generating a list of dates for every single visible date and checking if it is contained in that list, we now use a (admittedly scary-looking) predicate to compare the date to the start and enddates

src/main/java/ch/zhaw/gartenverwaltung/MyGardenController.java

@@ -108,7 +108,7 @@ public class MyGardenController {
                 stage.initModality(Modality.APPLICATION_MODAL);
                 stage.setResizable(true);
                 stage.showAndWait();
-            } catch (IOException | HardinessZoneNotSetException | PlantNotFoundException e) {
+            } catch (IOException | PlantNotFoundException e) {
                 // TODO: show error alert
                 LOG.log(Level.SEVERE, "Could not load plant details.", e);
             }