Scenario.simulate(): set initial values automatically if not specified

[lisphilar]

Summary of new feature

When we use SIR-F model for estimation and use SIR-FV model for scenario analysis, we need to specifiy initial value of Vaccinated for simulation with Scenario.simulate(y0_dict={"Vaccinated: 0}) in the previous versions.
In most cases, initial values are zeros.

Solution

PhaseUnit.simulate(y0_dict=None): when initial values were not specified, zeros will be used as the initial values rather than raising ValueError.