UserTraits

What: Serialize/deserialize calls are now also templated on UserTraits..., which is passed on to the base serializer. Some helpers for adding/removing and checking for the existence of trait are added as well.
Why: User traits allow arbitrary customization of serialization behavior based on user-defined terms. This enables more flexibility in checkpointing than an extra boolean variable, and is more clear in user-space code than a pre-defined variable name (EG: VT::RestoreProxies instead of s.isCheckpointRecovery())

Serialization overriding

What: When a non-intrusive serializer and an intrusive serializer are both found, call the non-intrusive serializer.
Why: Non-intrusive serializers can follow up with calls to the intrusive copy, meaning non-intrusive serializers can be used as a form of serialization subscriber. Particularly with user traits, we can define non-intrusive serializers which only run when a particular trait is used on a given object, allowing third parties (E.G. KokkosResilience) to perform custom logic before serialization of arbitrary objects. This can also be used to generally attach to serialization of objects for the purposes of tracing, etc.

For examples of these changes in action, see checkpoint_example_user_traits.cc/hpp