Restrict MCMC to sets with at least one observed element

The PR allows MCMC active/inactive set flag sampling only to those sets that have at least one observed element; the sets without observed elements are fixed in inactive state.

There are 2 advantages:
 * the parameter space of set states is dramatically reduced (for most of the real data sets); the MCMC sampler can explore the shrunk space much more effectively
 * previously the parameter `p` had to be constrained to small values to avoid degenerated solutions (`p` > 0.5 and many sets without observed elements are active); by disabling a priori unrelated sets we exclude the degenerated solutions, and `p` constraints could be relaxed