Disallow stack-allocated meshes

[mtao]

Every mesh should be initialized using a shared_ptr, as some datastructures (namely certain Attribute Handles) . This means that we need

• a static member function called create that constructs a std::shared_ptr<Mesh> (or for derive classes whatever dreived type).
• all constructors should be made private and friended with make_shared
• make the this can be checked by checking assert(bool(shared_from_this()));