Write the scene back to OPS + OPF files

[VEZY]

We should be able to write again a scene that we just read.

The only difference we should expect with the scene we read is that there will be as many OPFs as objects in the scene because we can't guarantee that they are the same (or it would be expensive to check that each node is the same...). This is in contrast of e.g. several plants that are defined by the same OPF in an OPS.

To do:

• Write an MTG that is a scene. Atm the MTG will be written as a very big OPF. We should write the OPFs with their original transformations by removing the scene transformation for each node of the OPF. The transformation is available at the scene root node. We also should write each OPF to a different file, using the original file names (available in the OPF node that is a direct child of the scene), or a proxy of it if the original file is used for several plants
• When writing the OPF back to disk, filter the ref_meshes to keep only the ones that are in the current OPF that is being written.