Generalised approach to perturbation variable conversion

[cemaccam]

A semaphore system could be used to signal availability of reference variable data, to determine when datasets containing perturbations requiring conversion are processed. Alternatively, reference data arrays could be exported to standalone NetCDF files. In either case, the existence of reference data could be checked when a dataset that requires it is accessed, and the CFizer can move on to the next file if it is not available. This would be a more robust, generalizable approach than specifying that, in this case, 3D files are processed last. It could also allow, in future, the option for previously-processed reference data to be in a different directory.