Initial subsimulator variable values are not available after `setup()`

[kyllingstad]

Let us say that we have an FMU instance with an output variable x which has a default initial value of 1.0. Furthermore, assume we have an object of type std::shared_ptr<slave> named instance which refers to this instance. Now, consider the following:

const auto xVarReference = cosim::find_variable(instance->modelDescription(), "x")->reference;
auto sim = cosim::slave_simulator(instance, "my_simulator");
sim.expose_for_getting(cosim::variable_type::real, xVarReference);
sim.setup({}, {}, {});
const auto xValue = sim.get_real(xVarReference);
assert(xValue == 1.0); // FAILS

The assertion in the last line should pass, but it will fail.

The reason is that we don't populate the output variable cache in slave_simulator in its setup() method. We should, because at the end of that method call, the (FMI 2.0) FMU instance will be in initialisation mode, where output variables are gettable.

[kyllingstad]

The fix is trivial: Just add the line

get_variables(duration::zero());

at the end of slave_simulator::impl::setup().

I'll do this while working on #756, as I'm making changes to slave_simulator and writing a unittest for it there anyway.

[markaren]

Potential fix for https://github.com/open-simulation-platform/libcosim/issues/609 I reckon

[kyllingstad]

Potential fix for #609 I reckon

I tried to run the quarter-car case with the new code in #765, and it seems unfortunately not. The line I added there to fix this issue only transfers subsimulator variables to the master so they're available there; it doesn't push them out to the subsimulators again.