Initialization of state in onnx fmu

[KristofferSkare]

Fixes #5

This PR introduces the possibility to initialize the state of the generated mlfmu using automatically generated fmu parameters from the interface json.

The interface is changed by expanding the InternalState filed to include an optional start_value. If start_value is set then parameters for initialization is generated for all the states corresponding to the agent_output_indexes. If initialization is used name arg also has to be set to something. This will be the name of the automatically generated parameter.

This is done by generating a parameter for each state that should be possible to initialize with a name given by the interface.
The generated parameters (FmiInputVariable) are given a new argument (agent_state_init_indexes) that specify with index in the state array that should be initialized with this parameter. This arg is set automatically from the interface.json. Since this is a arg of FmiInputVariable, any FMU input or parameter could, in principle, be used to initialize state. But for now there is no way for a normal fmu input/parameter to have this argument set to something. So a future improvement could be to open up the possibility of normal fmu inputs and parameters to be used to initialize state.

[Jorgelmh]

Great implementation! I like the possibility of implementing inputs as initializers for internal states, just need to evaluate how we can do this with the already existing implementation and logic behind FMU inputs (refer to my other comment).