multibody → third-party Relation

File in include/aligator/modelling/multibody	Includes file in include/aligator/third-party
frame-collision.hpp	polymorphic_cxx14.h
frame-placement.hpp	polymorphic_cxx14.h
frame-translation.hpp	polymorphic_cxx14.h
frame-velocity.hpp	polymorphic_cxx14.h