Improve description files

[louislelay]

There are some problems with the way the descriptions files are done. This is a proposal list of the todos in order to make them good enough.

• Add collision to the XML file
• Create some default tags to simplify the XML file
• Either make some simplified meshes for collision or use capsule/cylinder/boxes, it will make the training go faster. (almost done need to simplify the foot collision mesh)
• Makes all fixed files one big piece (not urgent while robot is wip but a good improvement for readibility)
• Export stl files on a meter scale and not a millimeter (more standard but not urgent)
• Export stl files with a more standard axis (don't remember what the standard way is but I'll ask around at pal)

(I'll take care of the 3 firsts are they're the most urgent, I just prefer to note it somewhere to not forget)