Graph.copy_property(): remove 'compatible graphs' semantics for edge properties

With this, edge properties with convertible types are always copied,
regardless if the edge set is the same between target and source
graphs. In case the edge set is not the same, the properties are copied
only for the edges in the intersection.

src/graph/graph_properties_copy.hh

@@ -214,7 +214,7 @@ struct copy_external_edge_property_dispatch
                     std::swap(u, v);
                 auto& es = tgt_edges[std::make_tuple(u, v)];
                 if (es.empty())
-                    throw ValueException("source and target graphs are not compatible");
+                    continue;
                 put(dst_map, es.front(), get(src_map, e));
                 es.pop_front();
             }

src/graph_tool/__init__.py

@@ -2727,6 +2727,24 @@ class Graph(object):
         from (defaults to the graph than owns `src`). If ``full == False``, then
         in the case of filtered graphs only the unmasked values are copied (with
         the remaining ones taking the type-dependent default value).
+
+        .. note::
+
+           In case the source property map belongs to different graphs, this
+           function behaves as follows.
+
+           For vertex properties, the source and target graphs must have the
+           same number of vertices, and the properties are copied according to
+           the index values.
+
+           For edge properties, the edge index is not important, and the
+           properties are copied by matching edges between the different graphs
+           according to the source and target vertex indexes. In case the graph
+           has parallel edges with the same source and target vertices, they are
+           matched according to their iteration order. The edge sets do not have
+           to be the same between source and target graphs, as the copying
+           occurs only for edges that lie at their intersection.
+
         """
         if tgt is None:
             tgt = self.new_property(src.key_type(),