Motion model, resampling criterion, sensor model

1. Adjusted parameters in the motion model to reduce noise.
2. Added a resampling criterion so that resample occurs when the entropy
of the weight vector equals to that when 95% of the probability mass are
distributed equally to 5% of the particles.
3. In the particle filter, 40 beams are taken as one observation.
4. (also in the particle filter) History of the particles are recorded
by weight = sqrt(weight)*p(z|x). Taking a square root reduces the
importance of past observations. (like learning for Markov decision
problems)