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

This is an implementation of the suggestion in #2870.
Maybe you can tidy up the logic a bit with something like:
```
units = self.latex.get('units', {})
units.update((x.name, x.unit.to_string(format='latex_inline')) for x in self.cols if x.unit)
if units:
    lines.append(self.splitter.join([units.get(x.name, ' ') for x in self.cols)])
```