Make data_product the unit for staging

[kzscisoft]

Currently the model for staging is to stage jobs (i.e. collections of code runs/other actions). For the purpose of working under a safe haven, it would be better to use data_product as the unit, and allow perhaps wildcards to be used to add everything under a job ID.

[kzscisoft]

@nathan-cummings-ukaea assigning you as we have pretty much done this now in your branch