Code cleanup and test coverage bump

[jacobwhall]

• Increasing safety of example_path fixture in tests/test_utils.py for repeated testing (65b286b)
• Removing make_dir function in utils.py in favor of more modern os.makedirs function call (4e9c76b)
• New test for rasterize function to check if it calls export_raster correctly when given an output path (65b286b)
• Reducing "nodata" value handling in export_raster for cleaner code and a test coverage bump (9a022d9)

More detailed information can be found in the commit messages and comments. This PR brings our test coverage up to 98%