Bom types: general usability improvements

[Andy-Grigg]

Originally raised by @ludovicsteinbach

• Some elements are defined as choice in the schema, meaning that one of the two (or more) element must be provided, but not both. This isn't apparent when using the Python types. When both are provided, the user gets an xmlschema error, which is quite detailed and helpful, but conceptually not at the right level, since the user is not manipulating XML.

• Python types do not reflect which elements are mandatory, as in "at least one item in the list". It's therefore possible to instantiate an almost empty Python BoM, and serialize it successfully, but deserializing it will fail, as all empty properties have been ignored during serialization.

Decide whether we want to address these issues and how