Speeding up optimize_splines

[izackp]

This seems to be the slowest part of the algorithm.. some ideas to speed it up (keep in mind I don't fully understand the workings of the algorithm):

Threading - Not sure if its possible, but perhaps its possible to do different sections of the image at a time.

Reducing iterations - The randomness of the optimizing seems to result in a high amount of iterations.. if there is a way to remove the random element from it then it should be possible to reduce the needed amount of iterations/guesses, as well as produce more consistent results.

I've already made a pull request that speeds it up quite a bit.