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.

@MichelJuillard I rebased to eliminate the conflict.

@MichelJuillard Is there anything still blocking this issue that I need to change?

I didn' t have time to check all the commits

@MichelJuillard @stepan-a This pull request in some form or another has been around for two and a half years now and there are many issues in the forum popping up that could be easily resolved with this functionality. Could we please soon pull this?