positions and scatter fixes and rework for meshes and primitives

The system was slightly reworked to manipulate primitives and regular meshes the simultaneously. The Gravity property of the Channel object is now the local position + the center of gravity of the mesh for uniformity.

Needs to be tested :

• Everything around the scatter system and cell positionning
• the plane cutting
• the infos viewing as text on cells
• anything else that might depend on positionning of cells

Also needs to be tested :

• the starting position of embryons, whether assetbundles, obj files or a mix of both in different time steps/channels (it needs to be centered properly)