Changes to speed up Earth relief and regions tutorials

[willschlitzer]

Makes the changes suggested for regions.py and earth-relief.py to reduce the time to generate the tutorials. This PR removes the internal national borders in regions.py and changes the land color to dark gray. It also changes the registration for the loaded Earth relief grid to "gridline" because that is the cached version.

Fixes#825

Reminders

• Run make format and make check to make sure the code follows the style guide.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[willschlitzer]

computation time summary:
- ../examples/tutorials/3d-perspective-image.py: 11.75 sec 0.0 MB
- ../examples/tutorials/regions.py: 10.57 sec 0.0 MB
- ../examples/tutorials/frames.py: 6.65 sec 0.0 MB
- ../examples/tutorials/lines.py: 6.62 sec 0.0 MB
- ../examples/tutorials/earth-relief.py: 6.43 sec 0.0 MB
- ../examples/gallery/grid/grdview_surface.py: 6.32 sec 0.0 MB
- ../examples/tutorials/text.py: 4.93 sec 0.0 MB
- ../examples/tutorials/contour-map.py: 4.61 sec 0.0 MB
- ../examples/tutorials/coastlines.py: 3.60 sec 0.0 MB

@seisman This doesn't appear to have changed the times to build the documentation; any ideas on how to improve it?

[weiji14]

Might need to merge in the change from #866 first to use the cache properly.

[willschlitzer]

computation time summary:
- ../examples/tutorials/3d-perspective-image.py: 10.86 sec 0.0 MB
- ../examples/tutorials/regions.py: 10.02 sec 0.0 MB
- ../examples/tutorials/frames.py: 6.39 sec 0.0 MB
- ../examples/tutorials/lines.py: 5.99 sec 0.0 MB
- ../examples/gallery/grid/grdview_surface.py: 5.84 sec 0.0 MB
- ../examples/tutorials/earth-relief.py: 5.63 sec 0.0 MB
- ../examples/tutorials/text.py: 4.36 sec 0.0 MB
- ../examples/tutorials/contour-map.py: 4.22 sec 0.0 MB
- ../examples/tutorials/coastlines.py: 3.27 sec 0.0 MB
- ../examples/tutorials/inset.py: 2.96 sec 0.0 MB
- ../examples/projections/azim/azim_general_perspective.py: 2.69 sec 0.0 MB
- ../examples/tutorials/plot.py: 2.48 sec 0.0 MB
- ../examples/tutorials/first-figure.py: 1.70 sec 0.0 MB
- ../examples/tutorials/configuration.py: 1.65 sec 0.0 MB
- ../examples/gallery/plot/inset.py: 1.39 sec 0.0 MB
- ../examples/gallery/grid/track_sampling.py: 1.11 sec 0.0 MB
- ../examples/gallery/coast/borders.py: 1.08 sec 0.0 MB
- ../examples/projections/cyl/cyl_cassini.py: 0.98 sec 0.0 MB

Merging the changes from #866 doesn't seem to have sped up the tests.

[seisman]

I'm afraid the key reason for the slow execution time is because there are too many figures in these tutorials (7 for 3d perspective figure and 11 for regions).

For example, in the 3d perspective figure, can we merge the last few (~3) figures into one figure?

[seisman]

Here is what we can do to improve the 3D respective tutorial:

1. Use a lower resolution earth relief grid (currently impossible, because lower-resolution grids don't support the region argument yet)
2. Merge the 2nd and the 3rd figures
3. Merge the last three figures (still easy to understand what contourpen, colorbar and perspective do).

[willschlitzer]

Closing this PR due to creating multiple branches and pushing them; discussion/further edits will be at #871