automate the testing of the jupyter notebooks

[wyli]

this ticket looks for an automated CI setup to ensure the quality of the notebooks.
see also discussions:

• Move jupyter notebooks out of the project-monai/monai repo MONAI#894
• https://github.com/Project-MONAI/MONAI/issues/872
• https://github.com/Project-MONAI/MONAI/issues/296
• Move jupyter notebooks out of the project-monai/monai repo MONAI#894 (comment)

[wyli]

#107 provides preliminary support for this feature