Remaining items on the `Schisto` module for if/when `Schisto` becomes a focus

[tbhallett]

Following the initial version of Schisto (which came in on #105), there are some remaining jobs to consider if/when this module becomes a focus.

• Examine prevalence of infection(s) over time
• Improve calibration overall (in the 'fitted districts' and others)
• Tests:
  • Check the HSI occurring
  • Check that MDA reduces burden
• Update document to remove out-of-date things about code/framework

[tbhallett]

@tdm32 -- I presume this is out-dated since you've done loads more work on Schisto. So closing this