Add ESM1.6 conversion driver to support single variable files

[blimlim]

The ESM1.6 spinup runs have so far been using the ESM1.5 conversion driver. This works without issue, as the UM output format is identical between the two models.

However, for ESM1.6 we would prefer year long, single variable files. We could achieve this by adding an ESM1.6 specific driver, which collects monthly files together, and sets up names appropriate for single variable files. We would also like to remove the extra atmosphere directory level for ESM1.6, which can also be implemented by the driver: ACCESS-NRI/access-esm1.6-configs#119

To simplify the review process, I think it makes sense to split this into two smaller PRs:

1. Move functions from conversion_driver_esm1p5.py which will be used across drivers into a drivers/common.py file. This would include things like functions for regex filename matching or for converting a generic list of filepaths. This might also be useful for future drivers like CM3 if we decide to implement them here.
2. Add an ESM specific driver. This will make use of the common functions, and include the required customisations for ESM1.6