[FEATURE]: FRONT3D Merge objects made of multiple parts

Some objects in the dataset are made of multiple parts/have erratic(strange) loadings. It would be neat to merge them and get a single object for every mesh loaded. In that way instance segmentation can work correctly, and the loaded world will have a greatly reduced number of parts.

Working points for this are either `_move_and_duplicate_furniture` or `_load_furniture_objs`.

A starting point could be #446.

Start by reviewing issue #446, then inspect the `_move_and_duplicate_furniture` and `_load_furniture_objs` entry points. Determine how multi-part objects are loaded and how they should become a single object per mesh. Done means merged objects support correct instance segmentation while reducing the number of loaded parts.

Written by the indexing model from the issue text.