Add resampling criteria

[zsunberg]

Particle filters need not resample on every step, we should add a resampling criterion so that resampling will only occur if e.g. the weights have a large amount of variation.

This would mean that the beliefs need to be weighted particle beliefs instead of just unweighted particle collections

[jihodori]

Hi Professor, I opened the following pull request to address this issue: #67

Let me know what you think!