add function to calculate DDD

[razekmh]

I would like to build on the work already done in processing the OMOP CDM to get the DDD. There's a function that computes DDD in RAMSES however the function requires:

• ATC_code: a character vector of ATC codes
• ATC_administration: a character vector indicating the ATC route of administration
• dose: a numeric vector indicating the total dose for the drug serving as the ATC DDD basis of strength. For prescriptions, provide the total dose to be given in one day, rather than per administration.
• unit: a character vector coercible to a units object with as_units.

Starting from OMOP data and getting to ATC codes is already done by @andysouth. I think I could build the rest of the table and either replicate the compute_ddd function from Ramses or build the table and call it.

The compute_ddd function calls the atc-ddd index offered by the WHO Collaborating Centre for Drug Statistics Methodology which could be parsed for the target drugs and stored instead of pinging the API for each instance. This risks stale data but we might refresh the data on intervals.