Option --mesh does not work with a file extension

Works:

```
precice-aste-run -p A --mesh input_mesh --data "scalars"
```

Does not work:

```
precice-aste-run -p A --mesh input_mesh.vtk --data "scalars"
```

Leading to:

```
---[ASTE:A:0] ERROR : Unable to handle basename input_mesh.vtk no meshes found
```

This is confusing and probably easy to handle.