Date slice

To start with I focussed on purely querying obersavtions to get out dates for a particular product. There is a lot here that can be cleand up, given some more time, but it's functional.

TODO: going to need pagination. Datasets are big and trying to return/render 1000s of observations in json is slow.

TODO: Factory girl repetition is a pain in the painful place. Can we just use the ones in models?