Implement simple function for efficacy based on vaccination history

[giovannic]

Inputs:

• Vaccination time
• gamma_v (the average duration of protection)
• gamma_vaccine_delay (the delay before protection starts)

The final functional form for efficacy is TBC.

current parameterisation current implementation

The new implementation will have to take into account the the combined effects of different vaccine types

[slwu89]

This has been fully addressed by #9 for the nimue vaccination model and #13 for a full model with Ab titre dynamics