first draft of fitness module, with just body-type-measurement for now.

> @context doesn't have to be specified or required here. It's handled automatically by rs.js core.

Yeah, I know, but I realized we hadn't been using `additionalProperties: false`, which makes everything much more prone to error because you could be passing in bad schema and it would likely not complain. 

I would like to make a proper fix for this in `remoteStorage.js` so that it will automatically add the `@content` property *before* it validates the schema, but this in the interim.