Add result_images folder to gitignore and make clean list

[weiji14]

Description of proposed changes

Tell git to ignore PNG files in 'result_images' folder, and add it to list of folders to clean for make clean. Patches #555.

Fixes #

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 docstrings or tutorials.