Use pytest's `tmp_path` fixture instead of `tempfile`

[Saransh-cpp]

https://github.com/search?q=repo%3Apybamm-team%2FPyBaMM%20tempfile&type=code marks all the occurrences of tempfile imports. These should be replaced with pytest's tmp_path fixture to maintain uniformity and reduce redundant imports.

[Prtm2110]

Hello, could you please checkout Prtm2110@d9adf56 and let me know if I am heading in the right direction.

[Saransh-cpp]

Hi @Prtm2110, it is usually recommended to open a PR as soon as you have something working, as that is much easier to review. Feel free to open one and I will review it as soon as possible :)

[Prtm2110]

Yes! I should have been more specific I was doubting the intended changes as although it is reducing imports but at the cost of readability I believe as we are just more often used with stdlib's tempfile and it's syntax.