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

I'm not sure about the timestamps here actually. The creation date is the ID of the object, so it's stored already, and for the update date: 1) one should probably not be able update measurements afterwards, except for correcting errors. 2) If it's just for correcting errors, do we ever need to know when it was fixed? I can't think of a use case for these 2 in the app we need this for, or any other app I can imagine at the moment.