IDEA: Add multiple CPU support to driver script (Performance)

[truth-quark]

Idea: if more performance is required, use the multiprocessing library to execute multiple conversion processes.

This is useful if there are requirements to convert many input files.

Code changes required include:

1. Splitting the input files into groups for faming out to different processes
2. Implementing worker processes
3. Reporting needs synchronisation to ensure the output logs are not interleaved

[aidanheerdegen]

Hannah reported that converting a year long run takes 15-20 minutes for "monthly files".

That does seem relatively sluggish. Parallelisation might be more effective, and perhaps more necessary?, when there is a single variable per file #89.

[truth-quark]

For reference, Hannah was converting:

• 12 * 512mb files
• 12 * 1.7GB files
• Total ~26GB converted in 15-20 minutes.

Adding multiprocessing should give good gains as files can be converted independently. Assuming ncpus=4, processing could be shrunk to ~5 minutes.