[ssn] editor's draft

I did a lot of writing on the editor's draft today - it still looks like notes but is getting close to covering all the issues in the "work plan" section. http://w3c.github.io/sdw/ssn/ So I am hoping little more is needed in that section, subject to  feedback.

I also wrote a lot about modularity there  -- things to be done rather than what has been done already - but feel free to appropriate  any of that into the modularity section as you see appropriate.

On the other hand, the Modularity section, I think,  needs to make very clear (write it as if it is final) description of how SSN is broken up now and how it interacts with dul and how the namespace/redirect is (intended to be) managed.

Does that sound ok?
Kerry