Fix ExclusionContainer.add_raster ignoring codes=0

[Blizzeq]

Changes proposed in this Pull Request

ExclusionContainer.add_raster(raster, codes=0) currently excludes every non-zero cell instead of the cells with value 0, because shape_availability checks the truthiness of codes and the integer 0 is handled like a missing argument. The same check breaks numpy arrays: np.array([0]) gives the inverted mask and a longer array raises a ValueError. I changed the check to is not None, so 0 and arrays now behave like the list form codes=[0]. One side effect to point out: an empty list used to fall back to "exclude all non-zero cells" and now excludes nothing. I also corrected the docstring, which said the default is 1 while it is None. The new test in test/test_gis.py needs no download and fails on master.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• Newly introduced dependencies are added to environment.yaml, environment_docs.yaml and setup.py (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.