Investigate dmd codes that are linked to a PF Consultation ID

[milanwiedemann]

We should start by implementing the same approach we used in the main https://github.com/opensafely/pharmacy-first project

• Count monthly medications of pharmacy first events pharmacy-first#54

This counts all the dmd codes in our pharmacy first dmd codelists, see here. We should follow this approach to also look at all the medications linked to a Pharmacy First Consultation ID that are not in our codelists, for example by using except_where(). This could be added as a new measure with the name nonpf_medication_count or something similar?