Make newTable, emptyTable flat. Fix flatten.

[devinrsmith]

Originally reported by @elijahpetty. Noticed that

t1 = newTable(
    stringCol("Title", "content"),
    stringCol("ColumnName", "column_content")
)

println t1.isFlat()

t1.flatten()

println t1.isFlat()

printed out false then true. This change makes sure that newTable and emptyTable produces an isFlat() Table out of the box, as well as ensuring that Table.flatten doesn't modify the existing table.

[rcaudy]

.