Add support for Jupyter notebooks

[AlexIoannides]

"As a ML Engineer, I would like to be able to define pipelines in Jupyter notebooks, so that I don't have to convert my notebooks into Python modules before I can deploy projects."

Tasks

• add nbconvert and nbformat packages to Bodywork requirements.
• in bodywork.stage_execution.run_stage, check the file type of the configured executable and if it's ipynb then run the notebook in a different path, as demonstrated here.
• check config validation to make sure that ipynb files will pass validation for executable files.
• add one unit test and one integration test for this use-case (will need new test resources).