Properly trigger pytest matplotlib image comparison

[weiji14]

Description of proposed changes

All the while, we weren't actually testing the image comparisons! I've actually had a hunch a long while back, but it was the change to the SRTM15+V2 grids since #350 that made me think things weren't right. There's also a bigger degree ° symbol now on the x and y axis for some of the plots using basemap.

Old	New	Diff

This Pull Request adds the --mpl flag and saves any different/failed results via the --mpl-results-path=results which will store the diff images under $TESTDIR/results. See upstream pytest-mpl docs.

I'll do this the Test-Driven Development (TDD) style, make the tests fail first (so we have a record), and then fix the images to make the tests pass. The failures come mostly from grdimage and grdcontour, but also makecpt and a couple of other random ones...

TODO:

• Trigger test failures (30 actual failures in total, ~35 images to change below).
• Fix grdcontour(5), grdimage (3), logo (2).
• Fix coast (4), image (1), makecpt (8)
• Fix basemap (4) and plot (8).

Fixes #

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.

[weiji14]

Ready for review! Mainly changes because of the new SRTM15+V2 grids, and some that have a bigger lat/lon degree sign on the axes.

[seisman]

Looks good to me.