BonePosFinder improved criteria for selecting lower and upper lines in ransac.

Now it uses the centroid of the inliers to decide which one is the lower and
which one is the higher.
BonePosFinder fixed bug in ransac method.

There was an error when calculating the wrist position that would change to
wrong positions paw wrist and elbow positions.
BonePosFinder fixed bug on sphere_line_intersection

The ecuation for sphere line intersection were not implemented
correctly.
BonePosFinder removed redundant calls to refine_start_position

Calls to this method have been centralized on find_leg_lower_3_joints,
so there is no repeated code in all the implementation methods. But there
were still a few of them left.
Added and removed several comments