void internalAssign(DocumentTypeManager other) {
        dataTypes = other.dataTypes;
        documentTypes = other.documentTypes;
        annotationTypeRegistry = other.annotationTypeRegistry;
    }
The DocumentTypeManager keeps track of the document types registered in
the Vespa common repository.

The DocumentTypeManager is also responsible for registering a FieldValue
factory for each data type a field can have. The Document object
uses this factory to serialize and deserialize the various datatypes.
The factory could also be used to expand the functionality of various
datatypes, for instance displaying the data type in human-readable form
or as XML.

@author Thomas Gundersen
document/src/main/java/com/yahoo/document/DocumentTypeManager.java