Add OP data on dm+d code usage

[emprestige]

Suggested addition to OCC to add usage information on dm+d code usage - as shown in openprescribing - thread here

Current OP usage limits users by having to type in drug names/chemicals, the ability to check usage for a codelist would be very good. While EPD reports slightly different data to that found in OS, an overview of high usage prescriptions when creating codelists/analysing medications data could be useful

[brianmackenna]

There is a plan (although unscheduled right now) to make this functionality available in OpenPrescribing V2.

(cc @richiecroker @iaindillingham for this slightly different description of the use case that we have already discussed)