improvement in Rays:
- rays.show3 now requires a layout as input (replace strucname)
- rays.show3 has now a center parameter (to center the layout )
- Rays has now a self.is3D, self.filled (interactions filled),
  self.evaluated (interactions evaluated)
- rays.show display the rays index if self.filled
bug fix in backtrace:
- add a tolerance on alpha and beta in signature.backtrace
This temporaly resolve the issue when a ray ouput into a corner.
To be updated when diffraction will be implemented
clear notebooks
Merge pull request #184 from niamiot/master

Imporvement in rays + bugfix in backtrace