Preparation for storage implementation of references.
Merge branch 'master' into reference-conversion

Conflicts:
	documents-core/src/main/scala/digitalistbesser/documents/format/FormatReader.scala
	documents-core/src/main/scala/digitalistbesser/documents/format/FormatReaderContext.scala
	documents-core/src/main/scala/digitalistbesser/documents/format/FormatWriter.scala
	documents-core/src/main/scala/digitalistbesser/documents/format/FormatWriterContext.scala
	documents-storage-orientdb/src/main/scala/digitalistbesser/documents/format/storage/orientdb/OrientFormatReaderContext.scala
	documents-storage-orientdb/src/main/scala/digitalistbesser/documents/format/storage/orientdb/OrientFormatWriterContext.scala
Rework of access to OrientDB documents in preparation of reference handling.
Rework of access to OrientDB documents in preparation of reference handling.
Added unit tests for default constraint of reference parts.
Rework of access to OrientDB documents in preparation of reference handling.
Rework of access to OrientDB documents in preparation of reference handling.
Rework of embedded collections. Some cleanup.
Implemented part builder trait for parts themselves.
Rework of handling of references between documents.