SL: Fix move down exception.

src/Model/SedimentLayer/SedimentLayer.py

@@ -284,7 +284,7 @@ class SedimentLayer(SQLSubModel):
         self.delete(els)
 
     def move_up(self, index):
-        if index < len(self._layers):
+        if index >= 0:
             next = index - 1
 
             l = self._layers
@@ -293,7 +293,7 @@ class SedimentLayer(SQLSubModel):
             self._status.modified()
 
     def move_down(self, index):
-        if index >= 0:
+        if index + 1 < len(self._layers):
             prev = index + 1
 
             l = self._layers