Added a NamedGroupType to separate them from unnamed groups

In the example in the Readme I will be able to change this 
```
    //filter out the LinkedTypes
      for(typ <- types.filter(p => p.isInstanceOf[LinkedType])){
        val linkedTyp = typ.asInstanceOf[LinkedType]
        val linkedTypName = linkedTyp.name.split("\\.")(1)
        if(linkedTypName == "color"){
          println("COLOR:\t" + linkedTyp.text)
        }
```

 to this 

```
    //filter out the LinkedTypes
      for(typ <- types.filter(p => p.isInstanceOf[NamedGroupType])){
        val namedGroupType = typ.asInstanceOf[NamedGroupType]
        if(namedGroupType.groupName == "color"){
          println("COLOR:\t" + linkedTyp.text)
        }
```