RNG Local Seeds

This is mostly a synced version of [a pull request](https://bitbucket.org/ompl/ompl/pull-request/32/rng-local-seeds) from BitBucket. Discussion in said pull request had favoured creating a "quasi-random" constructor for creating samplers of known seeds; however, I am unclear if that is the correct answer, as it:

1. Provides no method to set deterministic behaviour for RNGs themselves, only samplers (i.e., RRT's goal sampling would be unaffected)
2. Doesn't scale well to different types of state samplers ([a future pull request](https://github.com/utiasASRL/batch-informed-trees/tree/informed_sampling))
I believe this is done as a convenience to do deterministic goal sampling (see code below). We shouldn't require StateSamplers to have a random number generator.