* Moved `NCBITaxon` node to the `com.bio4j.model.nodes` package (removed `.ncbi` suffix);
* Same for `ReactomeTerm`: `com.bio4j.model.nodes{.reactome => }`;
* Separated `NodeRetriever` on a set of interfaces per a node type: `com.bio4j.model.util.<NodeType>Retriever`
* Made `NodeRetriever` an abstract interface dependent on the node type: `NodeRetriever<N extends com.bio4j.model.Node>`
what about this
Added NodeIndex
Merge branch 'feature/api/separate-node-retriever' of https://github.com/bio4j/bio4j into feature/api/separate-node-retriever

Conflicts:
	src/main/java/com/bio4j/model/util/NodeRetriever.java
fix merge conflict due to comments