Can't Serialize Custom Geometries

Hello,

I am trying to serialize my RTree of type using the following code:

`OutputStream os = new FileOutputStream(new File("test.tree"));
Serializer serializer = Serializers.flatBuffers().javaIo();
serializer.write(tree, os);
`

I keep getting the following error:

`
java.lang.RuntimeException: unexpected
at com.github.davidmoten.rtree.fbs.FlatBuffersHelper.addEntries(FlatBuffersHelper.java:60)
at com.github.davidmoten.rtree.fbs.SerializerFlatBuffers.addNode(SerializerFlatBuffers.java:96)
at com.github.davidmoten.rtree.fbs.SerializerFlatBuffers.write(SerializerFlatBuffers.java:73)
at CreateDataTests.main(CreateDataTests.java:31)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
`

It looks like the addEntries method of FlatBuffersHelper can only process the built in geometries. Is there a way around this?

Thanks