Run fmi2SetXXX function to initialize FMU start values

[davidhjp01]

According to FMI 2.0.4 spec (Section 4.2.4), start values are read during instantiated state via fmi2SetXXX . But libcosim does not run these functions in instantiated state and this makes some of FMUs exported (e.g. Simulink) not initializing values accordingly.

For example see below cosim_demo_app that runs an integrator whose initial condition is set via parameter Parameter.Integrator_x0:

The initial condition is not properly set via the parameter (i.e. not starting from 10).
On the other hand, fmpy properly initialize this value when set in the simulation setting:

To fix this, this PR added an additional function initialize_start_values() that runs before simulator::setup() in fixed_step_algorithm::cosim::impl::initialize()

Check state_init_test.cpp to observe different behaviours with the previous libcosim version

[davidhjp01]

I guess this PR contains some conflicts with #777, will need to update based on the merge :)

[kyllingstad]

The implementation looks good, and it's great to get this fixed. Correctness issues should always have high priority, IMO.

That said, I wonder if this really needs to be an entirely new interface function in simulator. Couldn't the things you do in the new simulator::initialize_start_values() function be done at the start of simulator::setup() instead, before it calls slave_->start_simulation()?

[kyllingstad]

variable_type and value_reference are just integers in disguise, and should therefore be passed by value rather than by reference.

[kyllingstad]

Also, we have the scalar_value alias which you can use instead of that variant.

[kyllingstad]

When adding an FMU, you should update tests/data/fmi2/README.md and possibly include a licence file, cf. #764.

[kyllingstad]

The functions are generally listed in the order they're supposed to be called, so this should come before setup().

[kyllingstad]

We haven't made reference to the FMI spec anywhere else in this interface, nor in any other of our general interfaces, because they are supposed to be agnostic with respect to the underlying binary interface. All FMI-specific stuff is hidden away in the cosim::fmi module. (I admit that we're still leaking FMI details like a sieve, but we haven't been this explicit about it before.) I suggest writing a more general and extensive description of what the function is supposed to do instead.

Also, as commented elsewhere, the functions are generally in call order, so this should go before setup().