code: @Nonnull
    public final HCColGroup addColumnAt (@Nonnegative final int nIndex, @Nullable final IHCCol <?> aCol)
    {
      if (aCol != null)
        addChildAt (nIndex, aCol);
      return this;
    }
docs: Add a new column.
  
  @param nIndex
  The index where the column should be added
  @param aCol
  the column to add
  @return this