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

This is an implementation of the suggestion in #2870.
Yes, please change the expected output of the tests.
LaTeX can encode symbols and formulas that plain ASCII cannot (e.g. the
A with a circle on the top that means Angstroem, or proper use of
exponents, try e.g. a unit like `u.km / u.s**2`). When we use a LaTeX
writer, we should make full use of astropy's ability to write the units
in LaTeX form with all the bells and whistles that normal ASCII output
does not have.