Update the BayesianProblem to use the new sampler

[amal-ghamdi]

Description:

Update the BayesainProblem class to have 2 behaviors. A default behavior to use the new samplers and a legacy behavior to use the old samplers (to ensure tests pass for now)

DoD:

• [x] The BayesainProblem is updated to have the behavior described above
• [x] Any related code is also updated (demos/tests)
• [x] The updates are reviewed and merged

Note:

• (written by @amal-ghamdi) notes from @chaozg in https://github.com/CUQI-DTU/CUQI-Book/issues/40 could be relevant
• (written by @amal-ghamdi) Suggestion by @jakobsj, Since the mcmc module is experimental, we can make the default behaviour of BayesainProblem for now to be the old behavior and pass a flag if we want the new behavior (@nabriis)