Multimethod for distance calculations.

The `get-distance-fn` multimethod dispatches based on identity to 
determine what distance function to use when calculating distances. 
By default the supported distance function keys are:

- :euclidean
- :manhattan
- :chebyshev
- :correlation
- :canberra
- :emd
- :euclidean-sq
- :discrete
- :cosine
- :angular
- :jensen-shannon

The default distance function is :emd which may not be appropriate for all
use cases since it doesn't minimize the variational distance like euclidean
would.  If you don't know why :emd is the default you should probably switch 
to using :euclidean.

Fast and Provably Good Seedings for k-Means is a paper by Olivier Bachem, 
Mario Lucic, S. Hamed Hassani, and Andreas Krause which introduces an 
improvement to the monte carlo markov chain approximation of k-means++ 
D^2 sampling. It accomplishes this by computing the D^2 sampling 
distribution with respect to the first cluster. This has the practical 
benefit of removing some of the assumptions, like choice of distance 
metric, which were imposed in the former framing. As such the name of 
this algorithm is assumption free k-mc^2. A savvy reader may note that 
by computing the D^2 sampling distribution as part of the steps this 
algorithm loses some of the theoretical advantages of the pure markov 
chain formulation. The paper argues that this is acceptable, because 
in practice computing the first D^2 sampling distribution ends up paying 
for itself by reducing the chain length necessary to get convergence 
guarantees.

Approximating K-Means in sublinear time is a paper written by 
Olivier Bachmem, Mario Lucic, Hamed Hassani, and Andreas Krause 
which shares a method for obtaining a provably good approximation
of k-means++ in sublinear time. The method they share uses markov 
chain monte carlo sampling in order to approximate the D^2 sampling 
that is used in k-means++. Since this method is proven to converge to 
drawing from the same distribution as D^2 sampling in k-means++ the 
theoretical competitiveness guarantees of k-means++ are inherited. 
This algorithm is sublinear with respect to input size which makes 
it different from other variants of k-means++ like k-means||. Whereas
a variant like k-means|| allows for a distributed k-means++ computation 
to be carried out across a cluster of computers, k-means-mc++ is 
better suited to running on a single machine.

A random initialization strategy for k means which lacks theoretical 
guarantees on solution quality for any individual run, but which will 
complete in O(n + k*d) time and only takes O(k*d) space.

K-Means clustering generates a specific number of disjoint, 
 non-hierarchical clusters. It is well suited to generating globular
 clusters. The K-Means method is numerical, unsupervised, 
 non-deterministic and iterative. Every member of a cluster is closer 
 to its cluster center than the center of any other cluster.

The choice of initial partition can greatly affect the final clusters 
that result, in terms of inter-cluster and intracluster distances and 
cohesion. As a result k means is best run multiple times in order to 
avoid the trap of a local minimum.

Simplification in this context means transforming a 
dataset that has duplicate values to one in which we 
there are no duplicates and one new column - the number 
of duplicates that were observed.