Use the xarray pcolormesh to plot grids

[leouieda]

It automatically adjusts the coordinates to match the center of each
pixel. Using matplotlib's pcolormesh would result in pixels being off by
0.5*spacing unless we have pixel registered grids (which we don't).

Fixes #147

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.