Assign authorship to "The PyGMT Developers"

[leouieda]

Replaces my name with "The PyGMT Developers" in authorship claims and
copyright notices. Include a definition of what that term means in the
AUTHORS.md file and include a reference to it in the README and doc
page.
Left my email on the setup.py because we don't have a project email
yet.

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 docstrings or tutorials.