The cropped meshes are now saved using data.save_mesh to allow for saving in npz and obj.

The meshes of some object for Challenge 1 Track 2 are not centered. This caused issues when computing view points. The meshes are now centered when converted to o3d format.
The output mesh is in the same position as the orginal one.

Only copy the data to be extracted. The previous strategy of copying the
original mesh in memory and deleting elements from it leads to slowdowns.