Create function exclude_potential_pcos()

[signekb]

Based on the functionality flow, this function should censor prescription purchases of metformin due to potential polycystic ovary syndrome (using sex and date of birth from BEF data).

osdc/vignettes/function-flow.Rmd

Lines 150 to 163 in 375e652

	### Metformin purchases for women below age 40
	The function `exclude_potential_pcos()` as input to exclude all
	purchases of metformin by women below age 40 (i.e., \<= 39 years old) at
	the date of purchase. It relies on REGISTER as input.
	This function contains two helper functions:
	- `keep_women()`
	- `drop_age_40_below()`
	<!-- TODO: Add details on how this filtering should be done -->
	<!-- TODO: Add which register this uses -->

In @Aastedet 's original script is it located at the top of population.R.

[lwjohnst86]

Include logic in algorithm.csv as well