Rename quantiles / values for consistency

[dajmcdon]

Creating an explicit issue for the second point in #147.

The decision is q -> values, tau -> quantile_values throughout. probs also becomes quantile_values. Any instance of levels was likely a mistake and should be quantile_values.

This will be breaking and require a version change.

[dajmcdon]

Also need to clean up the printing of dist_quantiles. Following some discussion (and investigating similar in the {distributional} package), I'm going to go with quantile(median)[n].