Introduce result-retention toggle

[MatNif]

Introducing a toggle to let the user decide if they want to prevent the results of a specific run from being overwritten by the next run.

• If the toggle is off (default) results will be saved in 'outputs/data/optimisation/centralised', which can be overwritten by new runs.
• If the toggle is on, the results will be saved in 'outputs/data/optimisation/centralised_run_X', where X=1,2,3,... depending on how many previous runs have been saved. I intentionally didn't go for the timestamp solutions because I wanted to keep naming as simple as possible, and the timestamp can already be found in the file explorer.

To test Run the optimisation script multiple times over, with the toggle activated or deactivated.

[ShiZhongming]

This PR has been tested. It run as decribed.