export of metamodel to Java

[NicolasAnquetil]

When exporting metamodel to Java, the inheritance between traits is not exported:

Trait << #FamixTType
	traits: {FamixTNamedEntity + FamixTReferenceable + FamixTSourceEntity + TEntityMetaLevelDependency};

gives:

public interface TType  {

whereas it should be:

public interface TType  extends  TNamedEntity, TReferenceable, TSourceEntity, TEntityMetaLevelDependency {

[NicolasAnquetil]

Currently all the recursively used traits are flattened in the classes using them, but it would be better to keep the hierarchy of traits.
For example, this would allow to use TPackage as argument of a methods that expects a TNamedEntity.
Currently we can use the class Package as argument because it declares implementing TNamedEntity, but we cannot use TPackage whereas we should be able to.