overwrite tag for flux.write

[rt17603]

Original report by Hannah Chawner (Bitbucket: [Hannah Chawner](https://bitbucket.org/Hannah Chawner), GitHub: hanchawn).

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Job scripts sometimes fail when the data needs to be written to a file, replacing the previous version. This is because user input is required to overwrite a file.

An overwrite=False tag could be added to flux.write to easily allow for overwriting files without the need for user input