Remove macOS DS_Store files

[MartinMSPedersen]

Thanks for this software.
This PR removes 23 .DS_Store files that macOS generates and should not be part
of any source code distribution.
It also adds .DS_Store to the git-ignore list.

[yaccob]

Just as a hint:

Files created by particular environments (operating systems, IDE, editor, ...) can (and should) be excluded in the environment that creates these files, rather than in the project's .gitignore.

Otherwise the project's .gitignore would need contain hundreds or thousands of entries to cover all these cases.

Environment specific exclusion can easily be achieved by editing to the git config like this ...
git config --global core.excludesfile ~/.gitignore
... and adding environment specific artifacts to this file.