Medulla features development - skinweight propagation

Continuing the development of more features for Medulla. This is a key feature that will allow changes to preserve skin weights while changing the skeletal structure. It works by propagating the previous weights through the new bones (more bones or less bones, it doesn't matter).

 Use cases examples:
 - When you find out that you need more joints for a part of the body that you have already weighted
 - When you want to paint the weights on a simple skeleton and then convert it to a more complex one
 - When you have already weighted a complex rig but you want a simpler one for real-time purposes
 
medulla - skinweight propagation - development preview from Marco Romeo on Vimeo.