getter and setter of vertex_array::m_primitives are not implemented.

[csukuangfj]

https://github.com/sque/glpp/blob/master/src/glpp/vertex_array.hpp

//! Set primitive type when drawing
void set_primitive(primitive_type mode);

//! Get primitive type when drawing
primitive_type get_primitive() const;

The above two member functions are not implemented.