977 remove t0 from simulation class and allow flexible start times in model

[annawendler]

Changes and Information

• The start time of the simulation is not assumed to be zero anymore but is determined by the initial flows given as input to the model.
• The parameter t0 was removed from the Simulation class.

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Closes #977

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.34%. Comparing base (a5cbb91) to head (59ffa22). Report is 8 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #979 +/- ## ======================================= Coverage 96.34% 96.34% ======================================= Files 129 129 Lines 10056 10071 +15 ======================================= + Hits 9688 9703 +15 Misses 368 368 ```

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[annawendler]

I see the problem that m_transitions can be changed without confirming that the last value there matches the last value in m_populations. I can implement a setter where we would reset m_populations accordingly. Then m_populations should be set private, too, which would making setting S and R for the initialization of the solver a bit more tedious. I'm not sure if this is worth it or if it would be enough to add a constraint to the check_constraints function. Another possibility would be to set m_transitions private anyway without implementing getter and setter. Then we could define a friend class to set the initial flows based on real data or need to find another solution.

[lenaploetzke]

I see the problem that m_transitions can be changed without confirming that the last value there matches the last value in m_populations. I can implement a setter where we would reset m_populations accordingly. Then m_populations should be set private, too, which would making setting S and R for the initialization of the solver a bit more tedious. I'm not sure if this is worth it or if it would be enough to add a constraint to the check_constraints function. Another possibility would be to set m_transitions private anyway without implementing getter and setter. Then we could define a friend class to set the initial flows based on real data or need to find another solution.

We discussed this and agreed to stay with the public members and implement an additional check in the check_constraints() function.