BoM types: implement missing items

[Andy-Grigg]

Originally raised by @ludovicsteinbach

• Annotations: can be a of two types (Text/Indicator)
  • It doesn't really fit with our current implementation of _list_props
  • Order is lost when xmlschema separates the two types in different lists of items, making round trips difficult.
• AnnotationSource/NonMIPartReference use an unspecified free-input XML element. We'll need to find a way to expose this in a way that's symetrical (serializing/deserializing) and that isn't too horrible in dealing with namespaces.
• Typical on a material's RecycleContent is difficult to handle: lack of element, empty element, element with any value of any type all have the same semantic interpretation. Also it's not yet used by the API.