Add global topography data and update gravity data

[leouieda]

Use a 0.5 degree ETOPO1 dataset as Earth topography (will be used in
isostasy examples and Bouguer gravity). Update the global gravity data
to use EIGEN-6C4 at constant height to avoid dealing with conversion
between orthometric and geometric heights. The topography grid is in
geometric heights converted using an EIGEN-6C4 geoid.

Add functions to load these datasets using Pooch and examples for each.
Add an example calculating the gravity disturbance using the global
gravity data.

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.

[santisoler]

The gallery is taking shape and looks good.
This PR is failing tests and docs building because the datasets cannot be found under https:/fatiando/harmonica/raw/master/data/.
If we are going to merge even with failing CI we should check that they don't fail on master afterwards.
Another way would be to create a temporary branch (and a temporary PR), change the datasets' url and then wait for CI to not fail.

Besides these observations, I'm happy with the shape harmonica is taking.

[santisoler]

Did you forget to add the datasets available?

[leouieda]

That README is what becomes the index page for sphinx gallery. So the examples are going to be rendered undetr that like so https://www.fatiando.org/verde/dev/sample_data/index.html

[leouieda]

This PR is failing tests and docs building because the datasets cannot be found

I forgot to add a line to the CI builds to copy the sample data to the local folder. We don't the CIs to be making downloads every time. The downloading itself is well tested in Pooch so there shouldn't be a problem.