[sampler] correct documentation

o [examples/pmc.py]: * change plot title "data" --> "weighted_samples"
                     * correct copy paste "mc" --> sampler
o [importance_sampling.py] docstings: "PMC-run"-->"importance sampling run"
o [markov_chain.py] remove whitespacess in call signature
[tools] add parallel_sampler

o include ``mpi4py`` in setup.py as requirement for extra 'parallelization'
o include example "pmc_mpi"