Option for chunking CMORISED files as in generating timeseries

@maritsandstad it would be good to have option like as we have during generating timeseries for creating cmorised files ; rightnow a single file is created which stores data for all months for that variable. It would be good to have option there we can generate cmorised files which has only 5, 10 or 20 .. years data which we can define.

Start by locating the code that creates CMORised files and the existing option used when generating timeseries. Trace how the requested year span is represented and how output filenames are chosen. Done means a configurable span such as 5, 10, or 20 years produces separate CMORised files with the corresponding data ranges.

Written by the indexing model from the issue text.