Add Graph.sizeLong() - to return the size of a graph (number of triples) as a long.

### Version

5.0.0

### Feature

`Graph.size()` returns an int.

Changing the return type is quite disruptive. It causes a java error on use of `Graph.size` assigned to be `int`.

Introduce `Graph.sizeLong` with the same semantics except it returns a Java long.

Provide a default implementation.


### Are you interested in contributing a solution yourself?

Yes