Redesign ModelBuilder

Move ModelBuilder to utils

 MeshPartBuilder: interface describing anything you can do to manipulate a meshpart (add vertex, index, box, etc).
MeshBuilder: implements MeshPartBuilder, allowing to create one or more mesh parts
ModelBuilder: reuses MeshBuilders for the same attributes (amongst multiple models), allow adding node and reusing parts. Subnoded to be implemented but should not be that hard.