[mix_adapt/pmc] bugfix, cleanup and added Rao-Blackwell sensitive test case

o removed unneccessary definitions from pmc_test (i.e. target, sampler)
o added samples from overlapping components such that the Rao-Blackwell
  option matters
o calculate_rho_.. and application of ``mincount`` criterion separated
o during the updates perform check "weights is None" only once