Added function to load ICGEM .gdf files

[santisoler]

Fixes #287

On this commit I've copied the load_icgem_gdf() function from pinga-lab/paper-moho-inversion-tesseroids. I've only added some documentation and fixed some PEP8 coding style.

I've also added some tests for the new function with a fake input file.

Maybe a Cookbook entry would be nice showing how to load an ICGEM .gdf file.

Checklist:

• Make tests for new code (at least 80% coverage)
• Create/update docstrings
• Include relevant equations and citations in docstrings
• Docstrings follow the style conventions
• Code follows PEP8 style conventions
• Code and docs have been spellchecked
• Include new dependencies in doc/install.rst, environment.yml, ci/requirements.txt.
• Documentation builds properly (run cd doc; make locally)
• Changelog entry (leave for last to avoid conflicts)
• First-time contributor? Add yourself to doc/contributors.rst (leave for last to avoid conflicts)

[leouieda]

@santis19 thanks for doing this!

I just merged #424 to get rid of the error in Travis. Could you please update your branch? Let me know when you're done and I'll merge it in.

[santisoler]

@leouieda done!

If it doesn't fail it's ready to be merged!
Thanks you for writing the function, almost a Copy&Paste PR 😬