Adding #include<memory> to use unique_pts

[ShaoxiongYao]

Hi,

Thank you for providing this tutorial! I tried to compile and run on my machine but realized that I need to add #include<memory> in include/rigidbody/RigidBody.h and include/viewer/SimViewer.h to use unique_ptr. Not sure if it is a problem of my g++ setup. I am looking forward to your reply.