add logvalue attribute to quantity

This PR does something quite straightforward: it adds a `logvalue` attribute to `Quantity`. 

While in general I wouldn't advocate having a whole set of attributes like this (e.g. `expvalue` or `lnvalue` or whatever), I thought adding this might be useful mainly because this is astronomy, and the first thing we generally do to a number is take the log of it :)

That said, I understand if it's judged that we're better off telling people to do ``np.log10(q.value)``.  This is pretty much syntactic sugar for that. But I think this is something that I would already use in my scientific code.

(cc @adrn @mdboom as the `Quantity` and `units` masters)