Provide tool to set up restart config

[sebhoerl]

It would be great to have a tool that takes an existing config file, an output directory, and then writes a new config that restarts that simulation. What do we need to take into account for that?

• Set firstIteration to the last iteration for which we have data in the previous run
• Optionally set lastIteration, but usually we just have a very high limit and let the convergence test do its job
• Provide the path to the convergence output, so the state can be retrieved
• Provide the path to the VDF flow (and travel times?) files, so the state can be retrieved.
• Provide the path to the population file from the last simulation

Anything else?