Continuous random element (r_prep) for all prep_any_strategy variables?

[jennifersmith203]

• prep_any_strategy 2 and 3 have been updated (28th Feb 24 PR "Update PrEP") to use a constant random element during each period of PrEP eligibility
• test the effect of making this update to all prep_any_strategy variables, specifically 4, 8 and 14 which are sampled when running core
• if we are happy with this we will add the change to core for all prep_any_strategy variables