WebPredict / MergeColumns / 0.1.0

Merge columns from from a collection of input rows into a designated column index, with an optional separator.

First parameter: input row data (array of rows)
Second parameter: list of column indices to merge (zero based)
Third parameter: optional destination index of merged columns (if null, appends to end of row as a new column)
Fourth parameter: optional string separator between merged column values