Add .ipynb to .gitignore and update contributing guide accordingly

[hakonanes]

Signed-off-by: Håkon Wiik Ånes [email protected]

Description of the change

Close #183.

Progress of the PR

• Docstrings for all functions
• Unit tests with pytest for all lines
• Clean code style by running black via pre-commit

For reviewers

• The PR title is short, concise, and will make sense 1 year later.
• New functions are imported in corresponding __init__.py.
• New features, API changes, and deprecations are mentioned in the
  unreleased section in CHANGELOG.rst.

[pc494]

Thanks for doing this @hakonanes, approved and merging.