Made Latex/AASTex tables use unit attribute of Column for output

This is an implementation of the suggestion in #2870.
Since the column unit here is nominally the same as the `latexdict` unit (both `s`) then this test seems a bit ambiguous.  Maybe you should set the column unit to something different for both `date` and `NUV exp.time` and verify the expected result.  In this case the `NUV exp.time` unit should still be `s`.