Consider making remaining carbon budget an external dataset created in workflow.prep.*

[MonikaFu]

          In general, I would prefer that this dataset is an external dataset created in `workflow.prep.*` and passed to the relevant function as it will change with each COP and I don't think it makes much sense for it to be an internal dataset in this package. But given the time sensitivity I'd leave it as it is for now and add an issue for the next time we touch this repo (if we are going to be running more COP projects with executive summaries in the future).

Originally posted by @MonikaFu in #327 (comment)