Adding multiple values to a property before setting the entity's id fails

[jbothma]

Steps to reproduce

entity = CompositeEntity(dataset, {"schema": schema})
entity.add(prop, [v1, v2])

Expected:

set(entity.get(prop)) == set(v1, v2)

Actual:

entity.get(prop) == [v1]

This is because the statements created by Enitty.add have id of None.

It's not clear from the documentation that the id must be set before adding values. Perhaps the documentation can improve, but maybe Entity.add should fail a bit more loudly too?