Standard maps should be reference maps

[Peter Kovacs]

Should we state in the graph structure concepts that node and arc/edge maps must be reference maps for all graph types?

It would make writing a general algorithm easier and more convenient since we have to use set() functions instead of operator[] only for the "external" map types, and all the "internal" maps, which are created by the algorithm could be used as reference maps. Maybe it could also improve the efficiency in some cases, but it is not clear and wasn't be tested.

Would this rule cause significant problems?