@Override
    public AccumulatorV2<ObjectWritable<A>, ObjectWritable<A>> copy() {
        return new MemoryAccumulator<>(this.memoryComputeKey, this.value);
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
@author Stephen Mallette (http://stephen.genoprime.com)
spark-gremlin/src/main/java/org/apache/tinkerpop/gremlin/spark/process/computer/MemoryAccumulator.java