Fix noisy DS_Store files for MacOS

[richardtallent]

Description

When using this repository locally on MacOS, the OS litters the repository with .DS_Store files, which contain nothing other than custom folder settings, such as view options.

This means that any MacOS user contributing to the repository will need to manually remove these files or at least ensure they don't become part of a commit.

I've added .DS_Store to the .gitignore file, which will resolve this.

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests

[w-e-w]

btw you could set up a git global ignore for your system

[viking1304]

I agree with both of you. I have it in global ignore, but I would prefer to also have it in the local ignore file - not because of me but because of others who do not have a global set.

[w-e-w]

clarify I wasn't disagreeing
it's a "by the way" for future reference kind of thing
global ignore .DS_Stor it's probably what every git user on Mac user shoud do

note I don't use Mac