randomisations - support new_seed argument

[shawnlaffan]

The randomisations currently override any seed argument if one or more iterations have already been run. This helps avoid accidental changes and makes replication easier.

However, for complex randomisations that are run from the command line using run_randomisation it would sometimes be easier to run one or more iterations in the GUI before dropping to the command line for parallel processing using multiple instances. This would mean the randomisation can be referred to by name and the various conditions do not need to be passed.