Remove warnings.
Change message in CannotOverwriteFileException class
Add ExtractException to extract methods in RDFDataRepository interface.
Update extract method in File_extractor module.
Update extract method in RDF_extractor module.
Formated classes in frontend.
Update implementation of extract methods - test mandatory fields.
Update call extracts methods using RDF repository in backends.
Add Extract/Transform/LoadException to RDFDataRepository interface.
Add nested mandatory checkout in LocalRDFRepo class.
Update calling in classes Local/VirtuosoRDF.
Update test in backend using repository.
Update method load in module File_loader.
Update method extract in module RDF_extractor.
Update method load in module RDF_loader.
Update method transform in module SPARQL_transformer.