Consolidate data and response function into single input type

[wtbarnes]

Carrying over a comment from #5 from @dstansby

A thought, there is a well defined mapping from observation > temperature response function, so maybe instead of

m = Model(data, kernel, temperature, model='nonexistent_model')

it could be

m = Model(observation, temperature, model='nonexistent_model')

where observation stores both the data and the temperature response functions (and possible errors on the data too?)

[wtbarnes]

May be related to #6 as well