Add convenience function Grounder.ground_best

[cthoyt]

There are so many places where I implemented a wrapper around Grounder.ground that returns just the first entry if any matches are returned, otherwise return None.

It seemed like a good idea to upstream it with some tests.

[bgyori]

Thanks this is great! We could also consider adding a ground_best function to api.py which calls this method of the default Grounder.