Make set_initial_flows() in IDE model compatible for both ConfirmedCasesNoAgeEntry and ConfirmedCasesDataEntry

[annawendler]

Currently, the function set_initial_flows() only works for the type ConfirmedCasesDataEntry which works for age-resolved data. In the IDE paper, we did a simulation using data that was not age resolved and used the type ConfirmedCasesNoAgeEntry. For this type, set_initial_fows() doesn't work anymore (since we implemented age resolution to the IDE model) which is what we want to fix here.