Allow other start days than zero in the IDE model

[lenaploetzke]

Motivation / Current Behaviour

Currently, we implicitly use that the starting day of the simulation is always zero and the last time in the initial data for flows is also always zero.

Enhancement description

Allow different values for the start day. Therefore we have to change

• the setting of the first time point for the populations in the constructor
• the description of the constructor
• setting current_time=-dt in the update of force of infection in the case of the simulation
• the parameter t_0 in the simulation files should be removed as it is are not used (already done in PR 793 add seasonality to ide model #972 )
• ?

Additional context

This may also affect #952.

Checklist

• Attached labels, especially loc:: or model:: labels.
• Linked to project