<<makeintodatatable name better when used with colspanbox

[vince_faller]

I want tableboxes that use ColSpanBox to name the columns better when interacting with `<<make into data table`.  Currently it just takes the lowest column name.  

dt = Open("$SAMPLE_DATA\Big Class.jmp");

new window("Row Borders", 
	tblbox = tablebox(
		ColSpanBox("1", 
			NumberColBox("age", dt:age << Get Values),
			StringColbox("Sex", dt:sex << Get Values), 
		),
		NumberColBox("Height", dt:height << Get Values)
	)
);

tblbox << Make into data table();

The behavior that tabulate does would be fine by me.   

(dt << Tabulate(
	Add Table( Column Table( Grouping Columns( :sex, :age ) ) )
)) << Make Into Data Table