Update tutorials

[mxochicale]

Outdated https://github.com/SciKit-Surgery/scikit-surgeryvtk/blob/master/docs/tutorials/overlay_window.ipynb which seems to use sksurgeryutils.common_overlay_app which might be a replication of https://github.com/SciKit-Surgery/scikit-surgeryvtk/blob/master/sksurgeryvtk/widgets/vtk_overlay_window.py

import sksurgeryutils.common_overlay_apps as coa
from PySide2 import QtWidgets, QtCore

• So, will remove /overlay_window.ipynb for future updates of tutorial section.
• It is not a good practice to add notebook without unit test (maybe later we can consider the use of nbmake )
• There are other tutorials that require to be reviewed and potentially updated https://github.com/SciKit-Surgery/scikit-surgeryvtk/tree/master/docs/tutorials
• https://github.com/SciKit-Surgery/scikit-surgeryvtk/blob/master/docs/tutorials/rendering_generator.rst
• https://github.com/SciKit-Surgery/scikit-surgeryvtk/blob/master/docs/tutorials/text_overlay.rst
• https://github.com/SciKit-Surgery/scikit-surgeryvtk/blob/master/docs/tutorials/voxelisation.rst