Use napoleon instead of numpydoc to unpin Sphinx

[santisoler]

The combination of latest sphinx (2.2.0), numpydoc, and sphinx_rtd_theme
causes a mangled rendering of the parameter list. We were pinning sphinx
to 1.8.5, which is getting old and causing dependency problems.
Replacing numpydoc with sphinx.ext.napoleon fixes the problem (though
generates slightly different output). Modify autosummary class in order
to generate a Methods Summary. Update Sphinx configuration: change
values of napoleon configuration to compile Returns and Attributes with
the same style as the Parameters list. The changes applied were based on
fatiando/pooch#122 and fatiando/harmonica#129.

Fixes #59

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.