Add effective sample size option for resampling

[osorensen]

ESS is defined from the weights. Let the user set a threshold below which resampling should be done. See, e.g., p. 417 in del Moral, Doucet, and Jasra (2006).

[osorensen]

Note that adding this option requires modification of code in various places, since we currently implicitly assume that all the weights are equal after the resampling step.