Fixed index error on fin leg positions in boneposfinder.

find_leg_lower_3_joints_simple was using length of the previous bone
instead of the current bone.
BonePosFinder added method to find leg positions using line fitting to the data.
CommonFitter refine positions methods deleted.

This methods had been moved to bonePosFinder and are no longer used in this
class or its derivates.
LegFitter included code to use BonePosFinder find_leg_lower_3_joints_line_fitting

This improved method gives in general better results than
find_leg_lower_3_joints_simple
SkeletonFitting deleted some no longer valid comments
RenderSkeletonization added method to draw lines