Taxonomy GI ids and old tax ids should be multiple-value properties

Here `.list()` thing is important as it's multiple-value property:

```java
graph.makeKey(NCBITaxonNode.GI_IDS_PROPERTY).dataType(String.class).list().unique().indexed(Vertex.class).make();
graph.makeKey(NCBITaxonNode.OLD_TAX_IDS_PROPERTY).dataType(String.class).list().unique().indexed(Vertex.class).make();
```

[TitanDB manual](https://github.com/thinkaurelius/titan/wiki/Type-Definition-Overview#single-and-multi-value-keys) for reference.

See ohnosequences/bio4j-scala#18