Deal with several incidences/prevalences for same time and location

[simonleandergrimm]

If there are several incidences/prevalences for a pathogen at the same time and location, pick the estimate that we are happiest with or combine estimates somehow.

[simonleandergrimm]

Way forward:

• Identify estimates where we need choose between estimates.
• Add a flag for such estimates.
• Create tests, that require us to specify our preferred estimate, for cases where there are multiple prevalences/incidences for the same pathogen, time, and location.

[simonleandergrimm]

Fixed by #152