Simplify use of the Triplestore triples() and remove() methods

[jesper-friis]

The Triplestore triples() and remove() methods were originally modelled after the corresponding methods in rdflib. However, they are unnecessarily inconvenient to use.

By changing the prototype from triples(self, triple) to triples(self, subject=None, predicate=None, object=None) would make the API more intuitive and easier to use. For instance, looping over all instances would simply be triples() instead of triples((None, None, None)). The same apply for remove().

The backend interface should be unchanged.

To ensure backward compatibility, we should allow subject to be either a string or a triple. We could also add an additional triple keyword argument. Providing a triple in subject or using the triple keyword argument should raise a deprecation warning.