V0.38.4

This is a relatively small release with a few nice enhancements and several bug fixes. There is also a documentation enhancement publicizing @ccleva's Parquet integration project.

Bug fixes

Fix bug where missing values in numeric columns could not be formatted. This enables arbitrary missing value indicators (e.g. "N/A") to be used in printing and saving to files.
@lwhite1

Replace parallelQuickSort with mergeSort (#968), to avoid incorrect sorting caused by race conditions when a custom sort object is used.
@lwhite1

fix issue #963 (#967) Relation.structure() fails for TableSlice with ClassCastException
@lwhite1

Enhancements

Aggregate by non-primitive column type that extends Number (#973), making it possible to add a column type for BigDecimal
@daugasauron
@Kallur

plotly - added range slider to Axis (#953) …
@smpawlowski

To support annotation in plot.ly javascript. (#944) …
@xcjusuih

Documentation

Added link to the tablesaw-parquet project in README (#966)
@ccleva