Minimal changes necessary to display prevalence reports. Closes #49

[mapesa]

@ereng Please review. Synopsis:

• Prevalence reports can be displayed for tests that have only 1 measure and whose results can be resolved as either Negative or Positive.
• The report shows a table showing counts and a percentage prevalence rate for each test. It also shows a time series of the prevalence rate.

[ereng]

Functioning fine.