Add uri() method to valuelist class

This is required if an InterpolationError is raised in a valuelist
method. The returned uri is a concatenation of the uri's of the
values in the valuelist.
diff --git a/reclass/values/valuelist.py b/reclass/values/valuelist.py
index b24bed3..8b8dd51 100644
--- a/reclass/values/valuelist.py
+++ b/reclass/values/valuelist.py
@@ -39,6 +39,9 @@
         self.assembleRefs()
         self._check_for_inv_query()
 
+    def uri(self):
+        return '; '.join([ x.uri() for x in self._values ])
+
     def has_references(self):
         return len(self._refs) > 0