Allow for setting a different time slice level for an entry in `process_availabilities.csv`

It may be useful for users to be able to specify in `process_availabilities.csv` that a particular entry should be applied to a different time slice level than the one implied by the `time_slice` column.

For an example, see my comment here: https://github.com/EnergySystemsModellingLab/MUSE2/issues/363#issuecomment-3426751620

This would be trivial to implement and would save users some typing. It's an open question as to whether this is a good feature to have though! It could be more confusing for users, so we should think carefully before we decide to do it.