Dataset interface changes

• rdfInterface\Dataset::map(), rdfInterface\Dataset::reduce() and rdfInterface\Dataset::forEach() take optional QuadCompare | QuadIterator | callable $filter = null parameter. As exactly the same filter parameter is already a part of many other dataset method signature it shouldn't make dataset implementation harder and it allows user to prepare much simple callbacks when quads filtering is needed.
• rdfInterface\DatasetCompare::every() signature is now the same as all other rdfInterface\DatasetCompare method signatures.
• rdfInterface\DatasetMapReduce::map() returns an instance of rdfInterface\DatasetMapReduce now.
• rdfInterface\DatasetListQuadParts interface added allowing simple ordered dataset traversal.