Log scale pickle

[pelson]

Fixes #1539.

[mdboom]

Manually merged as f4d3a1b

[pelson]

Thanks @mdboom. Do we want to head down the path of deprecating the use of the nested classes (via the aliases)?

[mdboom]

I think as an API, the nested classes are nice -- i.e. one would look for transforms related to log in the log scale's namespace, and the only reason we're doing this is to make pickling work... So I'm sort of 👎 on deprecating them. However, I could be convinced otherwise with the right argument.