Improvements for RANSAC

Backwards-compatible, same behavior as before.
RANSAC can tell with a probability >= stop_probability, that the currently sampled model is actually an outlier-free model, depending on the number of iterations and the current inlier ratio. There is no way to make this choice easier. But we could add a description to max_trials, that makes it easier for the user to set max_trials, such that at least one outlier-free set is sampled in one of the iterations (depending on an a-priori known inlier/outlier-ratio).