add survey by year to wallets for each cave: Charlotte's request

core/models/survex.py

@@ -246,6 +246,12 @@ class SurvexBlock(models.Model):
 
     def isSurvexBlock(self):  # Function used in templates
         return True
+        
+    def year(self):
+        if self.date:
+            return self.date.year
+        else:
+            return 1970
 
     def DayIndex(self):
         """This is used to set different colours for the different trips on