Add support for latexing array quantities

See #2258 for context - this makes it so that array quantities produce decent-looking latex output for ipython notebook consumption.

Look good to you in concept, @mdboom and @mhvk?  If so, I can add tests to check this properly
I'm not sure about a `to_latex` method here.  I think we should instead follow the example of `Unit` and have `to_string(format='foo')`, so that eventually we support all of the output formats that Units currently support.  That doesn't have to be part of this PR, but I don't think we should introduce a new public method here.