Wrap docstrings to 79 chars and check with flake8

[santisoler]

Wrap all docstrings to 79 characters of API functions. Set max-doc-length=79 characters to make flake8 check if any docstring exceeds it.

Fixes #58

Reminders

• [ ] Run make format and make check to make sure the code follows the style guide.
• [ ] Add tests for new features or tests that would have caught the bug that you're fixing.
• [ ] Add new public functions/methods/classes to doc/api/index.rst.
• [ ] Write detailed docstrings for all functions/methods.
• [ ] If adding new functionality, add an example to the docstring, gallery, and/or tutorials.