updated use case document

All ... I've begun adding use cases and requirements into the use case and requirements document<http://w3c.github.io/csvw/use-cases-and-requirements/> (source is on github(gh-pages) @ csvw/use-cases-and-requirements/index.html) starting with the surface temperatures databank use case<http://w3c.github.io/csvw/use-cases-and-requirements/#UC-SurfaceTemperatureDatabank>. I'll keep going in this style until told otherwise; feedback welcome. Regarding the requirements I've synthesised, this is only the very beginning - and I'm going to need help to extract, disambiguate and validate all the requirements we've been inferring so far.

Jeremy