Improve xml code to check for valid changes before starting

[durack1]

Compare list_outfiles and list_outfiles_paths to previous version of xml database prior to initiating xml creation - this way if no changes are to be made, the job will complete rather than loading up machines and the network over a weekend.

• Check for existing instance of make_cmip5_xml.py
• Consider situation where partial files were downloaded last run, and now complete - in such a case a new xml will be created
• Consider querying Jeff P on ways to interrogate his database to query new downloads, and also query new data publication (migration across filesystems)
• Will require that previous xml creation completed successfully, otherwise failed runs will continue to compare xml output file favourably

May need to interrogate Jeff Painter's database of files - which provides an update as to whether any new files (or partial files) have changed since the last run

[durack1]

@pochedls should this be reincarnated in cmiplib? If yes, I'll copy it across

[pochedls]

This is essentially what cmiplib does, so we can leave this.