add DS_store to gitignore

[aslgraefe]

.DS_Store
**/.DS_Store

git rm --cached -r .DS_Store
git commit -m "Ignore .DS_Store"

[VarenyaJ]

This is good - additionally, I have an alias in my ~/.bashrc: "cleaner"="find . -type d -name __pycache__ -exec rm -rf {} + 2>/dev/null && find . -name "*.pyc" -delete && find . -name ".DS_Store" -delete" which I run after formatting/linting/testing

[aslgraefe]

finished with #211 #208

[aslgraefe]

@VarenyaJ thanks, good idea, too!