Support for flatMap or arrayAgg

[niviksha]

Really loving Arquero so far! One thing I constantly run into is the need for a flatMap or equivalently SQL array_agg operation - Often grouping by a key, I want to collect the values into an array-valued output. I can use Javascript flatMap, but it would be great to have Arquero support this in its native ops

[niviksha]

Nevermind, I figured this out! This works perfectly

table.groupby('colA').rollup({ array_agg_col: d => op.concat(op.values(d.colB))})

[jheer]

For anyone who may come across this issue: As v4.0.0, Arquero has an array_agg aggregate function, whereas the values method now extracts the values of an object (similar to how keys returns its keys).