JoinRigidLink and SplitRigidLink to occur outside of the adapter

#### Description:

Since Multi-node links are not supported in SAP, we split any multi-node links into sub-links during the push, then join them up again if they fit the naming scheme on the pull. This causes some funny problems, such as #254. Instead, Join and Split should be exposed as public methods, and any multi-node links should fail to push and show a helpful error which directs the user to use SplitRigidLink() on the offending links.