Enable precomputed similarity matrices for clustering

[deargle]

Hi, I just opened PR #87. Context is that I had a precomputed distance matrix that I wanted to cluster with. I used T-SNE to scale it down to two dimensions for the filter function, but I didn't see a way to use metric='precomputed' with DBSCAN for the clustering with inverse_X because of the hypercube slicing making things un-square. So this PR is what I did to tell mapper to give a square matrix to the clusterer.

[sauln]

Thanks for working on this!

The distance matrix stuff hadn't been working for a while. Once the unit tests are passing and there are some new ones in place, I'll merge this is in.

[sauln]

#87 has been merged. Thank you!