Allow more vector types

[LeviTaule]

Using Boost.Geometry strategy, as described in Nexus' post.

Example:

namespace lib
{

// user can specialize this class to support other types
template <typename T>
struct vector_type
{
    static auto x(const T& v) { return v.x; }
    static auto y(const T& v) { return v.y; }
};

// convenience inside library, can also be in private namespace
template <typename T>
auto x(const T& v)
{
    return vector_type<T>::x(v);
}

}