public Builder vertexKeyTypes(final Map<String, String> vertexKeyTypes) {
            this.vertexKeyTypes = vertexKeyTypes;
            return this;
        }
Map of the data types of the vertex keys.  It is best to specify this Map if possible as the only
other way to attain it is to iterate all vertices to find all the property keys.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/io/graphml/GraphMLWriter.java