title: DBSCAN (with metric only) in scikit-learn
body: <p>I have objects and a distance function, and want to cluster these using <code>DBSCAN</code> method in <code>scikit-learn</code>. My objects don't have a representation in Euclidean space. I know, that it is possible to use<code>precomputed</code> metric, but in my case it's very impractical, due to large size of distance matrix. Is there any way to overcome this in <code>scikit-learn</code>?   Maybe, are there another python implementations of DBSCAN that can do so?</p>