Added routines that factor out posterior samplers into the iteration routine...

Added routines that factor out posterior samplers into the iteration routine posterior_sampler_iteration.m.
It generalizes the random_walk_metropolis_hastings.m structure but adds two new items:
- check_posterior_sampler_options.m which is a wrapper for method specific options (stored in options_.posterior_options)
- posterior_sampler_iteration.m which is a wrapper of the individual iteration of any posterior sampler.

It already embeds, with parallelization implementation:
- random_walk metropolis hastings
- independent metropolis hastings
- slice sampler

It also contains a quick fix for TARB, although the latter should be embedded in posterior_sampler_iteration.m as well.

Any new posterior sampler can be simply added to posterior_sampler_iteration.m, with  wrapper to specific iterations in check_posterior_sampler_options.m