In DM-14503 support for YAML serialization of PropertySet and PropertyList was added to daf_persistence. To allow gen3 butler to serialize these using YAML it makes sense to move the YAML support directly into daf_base. This would require a pyyaml dependency on daf_base but we already use yaml in many places and it's in the core python dependency list. YAML support could be optional in the sense that it is only enabled if yaml can be found.