675 alternative computation of compartments in IDE model (#702)

In order to reduce simulation time, compartment values in IDE SECIR model are now computed 
via last value and flow value of the current time step instead of historic flow values only.
For the initialization, compartment values are computed using the sum formula from the flows as before.
The order of the member functions is changed to clarify which functions are used to compute the
initial size of the compartments and which functions are used for later time steps.

Co-authored-by: lenaploetzke <70579874+lenaploetzke@users.noreply.github.com>
Co-authored-by: Martin J. Kühn <62713180+mknaranja@users.noreply.github.com>
Co-authored-by: René Schmieding <rene.schmieding@helmholtz-hzi.de>

Author: 4 people