WIP Add GRAV-D airborne gravity data from United States

[santisoler]

Add GRAV-D airborne gravity data from United States, made available by NOAA thought its website: https://www.ngs.noaa.gov/GRAV-D/

Couldn't find the license of the datasets.

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.