Remove core/reachable point distinction

In general, we don't care about core/reachable distinction, just about cluster/outlier distinction, so I'm simplifying the code to reflect this.

Everyone who wants to have back the distinction can checkout an earlier version, like v0.2.4: https://github.com/cloudwalkio/ddbscan/tree/03a67c1ab1c4eec017e183a9bf43d52b473b1f20