Auto update Job scripts when attributes are changed until files are already written.

[MikeDacre]

Right now jobs scripts are created by init. They should be stored in a format that allows them to be updated at any time prior to submission. i.e. changing the 'cores' keyword will update the script.

The best way to do this will be to move the string formatting into a write_script method of the Job.Script class that can be called any time to overwrite the current scripts.

[MikeDacre]

Fixed in 0.6.2