Add sampling for marginalised parameters

[thomasckng]

After the main sampling procedure, a feature for sampling the marginalised parameters using the posterior samples should be added.

[thomasckng]

User should be required to pass the prior to the *_marginalization argument when initialising the likelihood. This can separate the actual prior used in sampling and the prior for post-sampling. This extra prior can also be used when relative binning optimisation requires it.