Add functionality for computing posteriors

So far, we only have functions for computing the likelihood based on some simulation results (calculate_llh_for_table). It would be convenient to also have some corresponding calculate_posterior_for_table function. This should simplify, e.g., https://github.com/PEtab-dev/petab_test_suite/issues/60. Ideally, #311 should be implemented first.

Start by reading the existing calculate_llh_for_table function and reviewing prerequisite issue #311. Determine how a corresponding calculate_posterior_for_table function should fit the current simulation-results workflow; done means the function is available and can simplify the use case described in petab_test_suite issue 60.