SimpleMap and SimpleWriteMap
- Trivial adaptors, but they are useful in some case

Some combined maps will be reference map if the first
template parameter map is reference map or not. If I want
to give a refernce map as first map but there is a non
reference map parameter then I should wrap my first map
to a regular read-write map.

/// The namespace of LEMON

/// The namespace of LEMON

///

namespace lemon {

  /// The namespace of LEMON concepts and concept checking classes

  /// The namespace of LEMON concepts and concept checking classes

  ///

  namespace concept {}

}