Use `tempfile` for temporary JSON files.

[prady0t]

After merging #4125 , we generate a lot of temporary JSON files while running tests. These files are short-lived and are created and deleted with every test passing, although if the tests fail these files have to be deleted manually.

You can see the number of json files being generated when test suit fails many times :

We can use tempfile to generate temporary files to fix this. See : https://docs.python.org/3/library/tempfile.html

[prady0t]

You can assign this to me, I will open a PR soon.

[kratman]

This is true for notebooks as well. They make images and other files, but do not clean them up

[agriyakhetarpal]

For the notebooks, I think it's better to not use tempfiles so as to not mislead newcomers to PyBaMM and Python, plus users might want to inspect the images after the notebook/cell has been executed