Change OP maternity demographic growth to follow births like IP does

[DCEW]

A request from users via Karen.

[tomjemmett]

currently based on group column, and maternity is defined here by admission method, which isn't available for outpatients. not sure if there were any specific reasons for this (i.e., would it cause us issues to switch to using treatment function)

[tomjemmett]

group in IP is either elective (admimeth 1*), maternity (admimeth 3*), or non-elective (everything else).

group in OP is either procedure (has_procedure = True), first (is_first = true), or followup (everything else).

the demographics module runs for all groups, except maternity, and the births module runs just for maternity group.

so as it stands we cannot easily switch to births in OP because we would need to completely change how we flag and identify activity for both modules.

potential solution

introduce a new column is_maternity. for inpatients could be derived as above (admimeth 3*), or via tretspef (501, 560?), or both. For outpatients would have to be by tretspef (as before). for everything else (currently just A&E), would be false.

for demographics, the index would need to be is_maternity = False for all records, for births adjustment it should be is_maternity = True for all records.

[tomjemmett]

most of the logic for this will actually need to occur in nhp_data. introduce a new column called demog_type with values "births"/"demographics" to control which type of demographic adjustment to use

[tomjemmett]

closed by #633