Disease modules schedule events for specific years during "burn-in period" at initialisation

[joehcollins]

A number of modules (maternal health, schisto, EPI) schedule events to occur on a given year (pre 2023) during at initialisation which means that the start date for a simulation cannot be before the earliest of these years (which are hard coded) otherwise the model crashes from attempting to schedule events in the past. In the current version of master this means that you cannot run a simulation with a start date later than 2015.

This issue links to discussions within #1320 (@tbhallett)