Fix a bug with boundries in the RootMaterialWriter and RootMaterialDecorator.

Solve bug in RootMaterialWriter and RootMaterialDecorator. The name of the tree in which the material informations were stored was not using the boundry id, when multiple boundries were associated to a single volume only the first one would be taken into account. Solved by adding the boundry id to the tree name (name format : volume_boundry_layer_approach_sensitive).