Add statistics describing medications and conditions of PF consultations

[milanwiedemann]

I think the easist way to pull out these numbers is to write a new measures file that tracks the monthly changes in these statistics:

• Percentage of pharmacy first consultations with prescribed (pharmacy first?) medication
• Percentage of pharmacy first consultations with (pharmacy first?) clinical condition
• Percentage of pharmacy first consultations with (pharmacy first?) clinical condition AND (pharmacy first?) medication

We also wish to do some data exploration with the following statistics:

has_nonpf_condition - to look at patients with a PF consultation, but with a condition that is NOT part of the pharmacy first conditions codelist.

has_nonpf_med - to look at patients with a PF consultation, but with prescribed medication that is NOT part of the pharmacy first med codelist.

[viv3ckj]

We will not be undergoing data exploration as part of this issue and linked PR (#66), but will carry this out in sub issue (#67)