Implement MCMC stage

[cklyne]

Implement the following features: Given an input, a model and N sets of params, create an output distribution using MCMC.

[EmanuelSommer]

Just to be very specific the MCMC stage aims at estimating the distribution over parameters via samples from the posterior distribution:

• Specify prior + likelihood
• use the "model" the optimized solutions as the warmstarts of the K chains
• warmstart the NUTS sampler with the window_adaption then sample using NUTS (blackjax)
• these samples can then be used for prediction (note that for many samples and chains the memory requirements increase rapidly, so one should allow for compressed saving of those - maybe in chunks)
• Different kind of "predictions" are possible. Let's maybe discuss this in a separate meeting or let's create a separate Issue for that.