extra check for attaching leads

kwant/builder.py

@@ -1070,6 +1070,9 @@ class Builder(object):
         The lead numbering starts from zero and increments from there, i.e.
         the leads are numbered in the order in which they are attached.
         """
+        if self.symmetry.num_directions:
+            raise ValueError("Can only attach leads to finite systems")
+
         if add_cells < 0 or int(add_cells) != add_cells:
             raise ValueError('add_cells must be an integer >= 0.')