Fix serialisation for extensions

[igornikolic]

When using the Table extension to output a table object into a output file we get

Exception in thread "Thread-0" org.sim0mq.Sim0MQException: Unknown data type class org.nlogo.extensions.table.TableExtension$Table for encoding the ZeroMQ message
        at org.sim0mq.message.TypedMessage.encode(TypedMessage.java:353)
        at org.sim0mq.message.TypedMessage.encode0MQMessageUTF8(TypedMessage.java:182)
        at org.sim0mq.message.SimulationMessage.encode(SimulationMessage.java:78)
        at org.sim0mq.message.SimulationMessage.encodeUTF8(SimulationMessage.java:146)
        at nl.tudelft.tbm.netlogo_hpc.messages.MetricReportMessage.encode(MetricReportMessage.scala:44)
        at nl.tudelft.tbm.netlogo_hpc.NetworkConnection.run(NetworkConnection.scala:129)
        at java.lang.Thread.run(Thread.java:748)

netlogo-hpc does not now how to deserialize a Table object when writing into a file