* 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
experiment with different options for enhancing the model

Inside the `com.bio4j.model.experiment` package I've added 4 options
for encoding more at this level:

- `com.bio4j.model.experiment.enums` global enums for node and rel
  labels
- `com.bio4j.model.experiment.singletonEnums` singleton types for
  node and rel labels
- `com.bio4j.model.experiment.staticConst` just putting the `String`s
  corresponding to labels inside the static part of final classes
- `com.bio4j.model.experiment.superabstract` I like this. I think this
  is as far as you can go in Java. Nodes and Rels cannot be defined
  without specifying their type, its type is constrained to be
  represented by a singleton enum, and rels have source and target
  Nodes with precise typing.

@bio4j/coders feedback most welcome here. This is not urgent right
now, but it is something I want to set as soon as possible so that the
Java API stabilizes at this level (Bio4j 1.0?)
Reviewing and comparing the 4 proposed models
review changes, move Nodes to the superabstract alternative
Merge branch 'feature/more/expressive/model' of github.com:bio4j/bio4j into feature/more/expressive/model
refactor 3 rels as a sample
fix merge conflict due to comments
update index code, fix unneeded classiness
review changes, think about how to work with this, start with docs
write about indexes and generic retriever interfaces
Reviewing bio4j modules improvement proposals by @eparejatobes and commenting on it
think about the implications of module-level types
Merge branch 'feature/more/expressive/model' of github.com:bio4j/bio4j into feature/more/expressive/model
Fixed import of reactome
Updated the plugin version
Experimenting with properties
Updated NCBITaxon parent and Organism-NCBITaxon relations