Support Union types

Type inheritance implies polymorphism implies that a property may be any of type derived from the declared type. In some cases, a property may be able to be one of a discrete set of types.

For example, in the Schema.org schemas, many of the references can point to either a person or an organization. Person and Organization may derive from a common type, but other types derived from that common base type are not allowed.

There are several possible approaches for representing such unions:
1) Allow defining a discrete set of types for a property, that may or may not have a common base type, rather than a single type.
2) Define a "union" type structure that is an open complex type with properties for each "type"
3) Define an annotation that allows you to restrict the set of derived types of a property. The types must derive from the declared type of the property, which could be an abstract type (i.e., Edm.EntityType).