Use only strings in xarray.Dataset metadata

[leouieda]

The harmonica.load_icgem_gdf function populated the Dataset.attrs
with metadata derived from the file header. Two of these fields
(attributes and attributes_units) were lists. This was fine while
using xarray with netcdf4 but it does not work if trying to use scipy to
save/load the Dataset. And since xarray 0.11 stopped installing
netcdf4 automatically, we can't load our sample gravity and topography
data without adding netcdf4 as a dependency.

Instead of doing that, load_icgem_gdf now converts the metadata to
strings. New copies of the two sample grids have been generated using
scipy (netcdf3).

cc @santis19

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]

I'll take this into account for future implementations.
In my opinion, we should try to minimize the dependencies, and sounds like pydata developers share this thought. And for our needs it does not substantially affect our functions and how we will define Datasets.

[leouieda]

Yeah, this was a nasty surprise but it should be better now