text: Title: slice method returns None for mesh from stl model
  Body: vedo: v2014.5.1
  python: v3.9.18
  
  Why the slice method returns a `None` object for the _.stl_ model?
  
  ```python
  from vedo import *
  msh = Mesh("sample.stl").alpha(0.1).wireframe()
  mslice = msh.slice(normal=[0,1,0], origin=[0,0,0])
  mslice.c('purple5')
  show(msh, mslice, axes=1)
  ```
  
  I am using the following file:
  
  [sample.stl.zip](https://github.com/marcomusy/vedo/files/14302198/sample.stl.zip)
  
  Thanks!
  