Fix MeshElementGroup.copy() to return un-mesh-joined group

meshmode/mesh/__init__.py

@@ -86,6 +86,13 @@ class MeshElementGroup(Record):
             unit_nodes=unit_nodes,
             element_nr_base=element_nr_base, node_nr_base=node_nr_base)
 
+    def copy(self, **kwargs):
+        if "element_nr_base" not in kwargs:
+            kwargs["element_nr_base"] = None
+        if "node_nr_base" not in kwargs:
+            kwargs["node_nr_base"] = None
+        return Record.copy(self, **kwargs)
+
     @property
     def dim(self):
         return self.unit_nodes.shape[0]