Global gravity sample dataset

[santisoler]

Added a global gravity sample dataset as a netCDF file downloaded from ICGEM Computation Service.
The data contains the EIGEN 6C4 release gravity_earth field which is defined as the magnitude of the gradient of the potential (including the centrifugal potential) at a given point.
It was calculated on the Earth's surface and on a regular global grid of 1.0° spacing.

Fixes #9

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.

[stickler-ci]

Could not review pull request. It may be too large, or contain no reviewable changes.

[leouieda]

@santis19 thanks for working on this! Please have a look at how I did this in Verde.

A few things that need to be changed:

• Data should go in the data top-level folder
• Add a README.md to data (see verde). It should contain a list of data files with explanations and attributions.
• Try using compression to reduce the size of the data file as much as possible. See if you can get it down a bit from the current 2mb.

We can merge this without an example or loading function. Those can come in a later PR.

[stickler-ci]

Could not review pull request. It may be too large, or contain no reviewable changes.

[santisoler]

@leouieda
See if it's good enough (and the README.md is well written).
The compression significantly reduced the file size (from 2mb to 372kb).

I think this PR is ready to be merged.

[leouieda]

Perfect!