Add PAM algorithm...?

[mthelm85]

I've been attempting to implement PAM in Julia for the past few days and I have a successful implementation that you can view here, along with some cluster quality comparisons (via silhouette scores). To sum it up, my PAM.jl package achieves higher silhouette scores than kmedoids (at the expense of performance) but not as high as those achieved by the pam function implemented in R's cluster package.

I'm opening this issue to see if there's any interest in adapting the code to make it work with Clustering.jl. If so, I'll work on it and submit a PR (WARNING: I'm inexperienced at package development). Also, for reference, I've found some literature that discusses parallel implemenetations of PAM:

• Schubert & Rousseeuw
• Mushtaq et al.

I probably don't have the experience/Julia knowledge to implement a better version than what I have here but I'm more than willing to invest time into it if there's interest in adapting for Clustering.jl and implementing a faster/parallel version.