Added UI elements to add/remove Scenarios to various objects (#51)

Fixes #23

Co-authored-by: Tim Lorsbach <tim@lorsba.ch>
Reviewed-on: enviPath/enviPy#51

epdb/models.py

@@ -534,6 +534,16 @@ class AliasMixin(models.Model):
 class ScenarioMixin(models.Model):
     scenarios = models.ManyToManyField("epdb.Scenario", verbose_name='Attached Scenarios')
 
+    @transaction.atomic
+    def set_scenarios(self, scenarios: List['Scenario']):
+        self.scenarios.clear()
+        self.save()
+
+        for s in scenarios:
+            self.scenarios.add(s)
+
+        self.save()
+
     class Meta:
         abstract = True