Made the return type of get_texture_coordinates const ref

In PcaModel, get_triangle_list returned by value before, we changed that recently, because if called in an inner loop, it was quite time consuming. So I think it's best to also return by const-ref here.

include/eos/morphablemodel/MorphableModel.hpp

@@ -397,7 +397,7 @@ class MorphableModel
      *
      * @return The texture coordinates for the model vertices.
      */
-    std::vector<std::array<double, 2>> get_texture_coordinates() const
+    const std::vector<std::array<double, 2>>& get_texture_coordinates() const
     {
         return texture_coordinates;
     };