Mapper algorithm using dbscan

[schnirz]

The dbscan algorithm is a nice candidate for the clustering algorithm used in Mapper, as it doesn't require an a priori specification of the number of clusters or additional computational steps like elbow detection. An implementation of dbscan in Julia already exists (http://juliastats.github.io/Clustering.jl/stable/dbscan.html), so using dbscan in Mapper should be rather straightforward.
Thoughts?

[wildart]

Good idea. I need to refactor interface to skip the optimal clustering selection step for clustering methods that doesn't rely on it.