Wrap docstrings to 79 chars and check with flake8

[leouieda]

Wrap all docstrings to 79 characters of API functions. Exclude examples
(docstrings are rendered by sphinx-gallery). Set max-doc-length=79
characters to make flake8 check if any docstring exceeds it. The setting
checks comments as well and we found no way of disabling that. So we'll
format comments to 79 characters for consistency as well. Minor
modifications to the first line of some docstrings was required to make
them fit into a single line. Also made minor modifications on some
doctests to wrap to 79 characters.

Fixes #207

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 and verde/__init__.py.
• Write detailed docstrings for all functions/classes/methods.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.