fix StructuredLine fallback boundary names

The `StructuredLine` function defines duplicate boundary names as default. When
requesting the boundary of the structured line, an attempt is made to create a
`DisjointUnionTopology`, but this fails because the names are duplicate. This
patch fixes the problem by defining distinct names as default.

nutils/topology.py

@@ -847,7 +847,7 @@ def __init__(self, trans, opposite):
 
 def StructuredLine(root:transform.stricttransformitem, i:types.strictint, j:types.strictint, periodic:bool=False, bnames:types.tuple[types.strictstr]=None):
   if bnames is None:
-    bnames = ('_structured_line_dummy_boundary_name_',) * 2
+    bnames = '_structured_line_dummy_boundary_left', '_structured_line_dummy_boundary_right'
   return StructuredTopology(root, axes=(transformseq.DimAxis(i,j,j if periodic else 0,periodic),), nrefine=0, bnames=(bnames,))
 
 class StructuredTopology(Topology):