3d tiles export implementation

[PerspectivesLab]

Hello,

I implemented a 3D tiles export option for nxsbuilder. It is currenty one of the most used 3D format based on multi-resolution so it might be useful to someone to have this option.
Just use the parameter '-e' to get a 3d tileset instead of a nsx file. I tested this on all meshes from Nexus_test folder and it seems to be working just fine. If you want to visualize the result, here's a repo to set up a 3d-tiles viewer https://github.com/charly-perspectives/3D-tiles-viewer. Thank you for reading me