nearest neighbors geom (and nearest neighbors option for disk geom)

[corybrunson]

To my knowledge, no {ggplot2} extension provides a statistical transformation layer to compute the nearest neighbors (NN) of (a subset of) the points in a 2-D cloud. For the following reasons, i think this should be added to {ggtda}:

• NN selection is a topological operation, so NN prediction is a topological tool (it is "within scope").
• A primary purpose of this package is to illustrate topological tools on 2-D data (it would fit in with existing functionality).
• NN engines might support future illustrative layers like neighborhood-based disks and witness complexes or even computational layers like witness complex--based persistent homology (it would support additional more obvious extensions).

I think this can wait until after the first CRAN submission, so i raise the issue as a reminder and to invite feedback.