Refactor sampler warnings to a serializable data structure

[michaelosthege]

As discussed in this and this comment the sampler warnings should be refactored into a data structure that can accomodate...

• Hierarchical samplers
• Samplers of different type (not everything is NUTS)

This is mostly a matter of thinking (hard) about data structures, also with McBackend protobufs and ArviZ InferenceData schema in mind.