Method to check if attributes were set explicitly

As requested by @taldcroft in #6, this adds a frame method and associated machinery that checks if a frame's attributes were set explicitly by the user, or provided as a default.

Note that initially I was thinking to have the `None` value internally mean default, but that doesn't work because in some cases the attribute might actually be `None` (e.g., for `obstime` it means "whatever equinox is")

There are a few other typos/minor fixes I added in the process of doing this that are also in this PR.