Add functionality Prior posterior function

Implements the functionality for #332 (closed) following the discussion in #417 @houtanb Still missing is the preprocessor implementation that translates a call like posterior_function_results=execute_prior_posterior_function('posterior_function_demo','posterior') into

posterior_function_results=execute_prior_posterior_function('posterior_function_demo',M_,options_,oo_,estim_params_,bayestopt_,dataset_,dataset_info,'posterior')

The only option should be prior_posterior_sampling_draws.