code: public ExtendedRelation addFeaturesRelationship(
  			String baseFeaturesTableName, String relatedFeaturesTableName,
  			UserMappingTable userMappingTable) {
  		return addRelationship(baseFeaturesTableName, relatedFeaturesTableName,
  				userMappingTable, RelationType.FEATURES);
  	}
docs: Adds a features relationship between the base feature and related feature
  table. Creates the user mapping table if needed.
  
  @param baseFeaturesTableName
  base features table name
  @param relatedFeaturesTableName
  related features table name
  @param userMappingTable
  user mapping table
  @return The relationship that was added