Remove subfix new and move mcmc out of experimental

[chaozg]

related to #40 from CUQI-Book

TLDR:
cuqi.experimental.mcmc.NUTSNew -> cuqi.mcmc.NUTS

In details:

• removed subfix "New" from names of samplers at experimental.mcmc: moved to Remove postfix New (only) from sampler names #477
• moved mcmc module out of experimental
• experimental module is left there for future experimental work though it's empty now
• updated tests/zexperimental/test_mcmc
• updated new sampler-related demos: demo36 and demo37

@nabriis suggested:

• I think we should check the feasibility of using the new samplers in BaysianProblem & the training material before moving it out of experiemental module
• If we move it out of experimental, I think we need to move the sampler module into somewhere like "legacy"