Should random walk functions produce more than walk_number, x and y

[spsanderson]

Should random_walk functions create columns more than walk_number, x and y?

[spsanderson]

As discussed with @AnttiRask I will include the following columns as defaults for all functions that generate random walks with the exception of rw30()

1. cum_sum
2. cum_prod
3. cum_min
4. cum_max
5. cum_mean

[spsanderson]

done

[spsanderson]

@AnttiRask I am going to add cum_min, cum_max and cum_mean to the brownian motion functions, cum_sum is y and cum_prod would just be exponentially huge very quickly so it does not make sense.

[spsanderson]

This is done with #57