Add tree.step to the repository

[nilswagner]

The file tree_structure_mini.uff
refers to a CAD file called tree.step
Can you add that file to the repository for completeness?
It might interesting to create a corresponding finite element model based on that CAD file.

Name: 3D tree structure
Description: Dimention: 379x179x474 - CAD model: tree.step

[jankoslavic]

Lovro, can you please handle this issue by uploading the step file?