Add grouper module
Add grouper module

This provides a class which handles aggregating practice level data into
higher organisation levels e.g. CCG or STP

ebmdatalab/openprescribing,annapowellsmith/openpresc,annapowellsmith/openpresc,ebmdatalab/openprescribing,annapowellsmith/openpresc,ebmdatalab/openprescribing,ebmdatalab/openprescribing,annapowellsmith/openpresc