feat: add support for drawing velocity vectors

include/screen.hpp

@@ -51,8 +51,11 @@ struct Screen {
     ~Screen() { delete[] buf; }
 
     std::vector<const vec<2>*> points;
+    std::vector<std::pair<const vec<2>*,const vec<2>*>> vecs;
     char_shader_t shader;
 
+    bool draw_vecs = true;
+
 private:
     Symbol *buf;
     Symbol _dummy;

include/swarm.hpp

@@ -132,6 +132,7 @@ struct Particle : public Agent<vec<N>> {
     }
 
     const vec<N> &get_position() const { return position; };
+    const vec<N> &get_velocity() const { return velocity; };
 
 private:
     A alg;