Rust (1987) -- data generation (runs)

* Regenerative random walk between mileages
* x_t can either stay the same or increase by 1; theta_3 is the probability that x_t increases (same for all x_t)
* Structural errors follow standardized Gumbel distribution
* Optimal control found via value function iteration
* Cost function assumed to be cubic
* To run, call generate_data; you'll get a list of tuples of numpy arrays
Merge remote-tracking branch 'origin/master'