Add support for multiple epidemics

[spixi]

The current implementation of DP-3T assumes that there is only one epidemic. However, the DP-3T protocol could also be used for other high-infectious patheogens, for example influenza, measles or smallpox viruses. Some pathogens are harmless by themselves but cause superinfections in presence of others (e. g. the hepatitis D virus is only dangerous in combination the the hepatitis B virus; this is just an example, because a droplet infection is very unlikely).

In my opinion, it should be possible to track different types of epidemics. This requires to add a list of epidemic ids for each EphID.

[spixi]

To mark the occasion: A study about a new H1N1 (swine flu) strain has been published recently: https://www.pnas.org/content/early/2020/06/23/1921186117