Add `to` method to table columns

This PR adds a new meth to table columns called `to`.  It's analogous to the `to` of `Quantity`, in that it returns a `Quantity` object in the requested units.

It also works as an "easy" way to get out `Quantity` objects if you do ``.to(None)``.  I say "easy" because it's really just syntactic sugar for u.Quantity(table['colname'])... But I find the latter to be more awkward to use in regular code, so I like the option to call a method that yields a Quantity.  And the unit-conversion is of course valuable in various circumstances.

cc @taldcroft @astrofrog @mhvk
Still another "an" -> "a".  sorry... (though I did say "twice" in my original comment...)