Added 1 line to fix error in loading Stl meshes

[p-shg]

Ran into the following error when using a BioMod linked to STL meshes

  File "/home/pierre/miniconda3/envs/pyorerun_sandbox/lib/python3.12/site-packages/pyorerun/model_components/mesh.py", line 18, in __init__
    mesh.metadata["file_name"] if "file_name" in mesh.metadata else mesh.metadata["header"].replace(" ", "")
                                                                    ~~~~~~~~~~~~~^^^^^^^^^^
KeyError: 'header'

This PR fixes that problem

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This change is 

[Ipuch]

That type of PR 😎😎

[Ipuch]

@p-shg black your code and it should be good to me :)

[p-shg]

Sorry I thought I did, should be good now