Allow load_icgem_gdf to take open file objects

[leouieda]

Previously just assumed fname was a string and called open on it.
Now, check if it's a file-like object (has a read method) and don't
open it if that's the case. Use contextlib to keep the with
statement in case we do have to open the file. This is useful if the
file is compressed and can be opened with bz2.open, gzip.open, etc
before passing on to the function.

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 and the base __init__.py file for the package.
• Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.