Fall 2019 release

We've made two major updates for this release.

The centroid functions have gained a keyword argument, squared. The final
step in computing distance to group centroids involves taking a square root.
Sometimes, we end up with a negative number inside the square root. Normally,
this produces NaN as a result. However, if squared is set to TRUE, we
don't take the square root and the result is always a real number. The default
setting is squared = FALSE, which gives the distance as you'd expect. Thanks
to Sam Ross for helpful advice on this topic.

We added a new function, pivot_to_numeric_matrix(). This function takes a
data frame in long format and converts to a matrix suitable for distance
calculations. Long-format data frames are commonly used with functions in the
tidyverse, and proper conversion to a matrix requires a few non-obvious
steps. The packages dplyr, tidyr, and tibble are needed to run the
function, and have been added as suggested packages for usedist.

During development, we had implemented an additional function to create a
distance matrix directly from a data frame in long format. However, we found
that it was nearly as convenient to use pivot_to_numeric_matrix() and
dist_make() together to achieve the same result. We added an example to the
README file to illustrate this.