restructured modules

o moved importance_sampling/markov_chain --> new submodule "sampler"
o renamed cluster --> mix_adapt
o moved proposal in markov_chain and in importance_sampling to new
  submodule "density"
o moved cluster.mixture_factory to density.mixture
[markov_chain] catch invalid start

o initializing a markov chain at a point where the target is not finite
  (i.e. nan or +-inf) raises ValueError now.
o hereby: better efficiency because the target is now evaluated only once
  per point