Simplify sanple representation

[cscherrer]

@sethaxen suggests

@cscherrer it likewise would be nice to move the conversion to InferenceData from SampleChains out of this package. This could either live in SampleChains/SampleChainsDynamicHMC via (conditional) dependence or in a glue package that users would need to import. What do you think?

Originally posted by @sethaxen in arviz-devs/ArviZ.jl#239 (comment)

I've been wanting to refactor these things for a while. Currently SampleChains is much too heavy, and adding new samplers is much too awkward. Maybe it should really be more of an interface, like

1. We need to know how to summarize results and print to the screen
2. There needs to be a function for converting samples into ArviZ.jl InferenceData
3. ....