template<typename P>
struct boost::gil::Point2DConcept< P >
2-dimensional point concept
concept Point2DConcept<typename T> : PointNDConcept<T>
{
where num_dimensions == 2;
where SameType<axis<0>::type, axis<1>::type>;
typename value_type = axis<0>::type;
value_type const& operator[](T const&, size_t i);
value_type& operator[](T&, size_t i);
value_type x,y;
};