cleanup declaration of ModelInterface's SharedPtrs
I propose to declare the SharedPtrs for ModelInterface together with all other model-related types in urdf_model/types.h instead of urdf_world/types.h.
Currently, even after having included urdf_model/model.h, one also needs to include urdf_world/types.h to get the related SharedPtr definitions.
With this PR, it suffices to include urdf_model/types.h to get all declarations or to include urdf_model/model.h for all declarations and definition of ModelInterface.
I'm going through my open PRs and just found this 6-years old one, which was already kind-of approved by @sloretz.
I rebased and retargeted against master and kept the file urdf_world/types.h with a deprecation warning indicating to switch to urdf_model/types.h.
Maybe in this form, it can be merged?