Eliminate compiler warnings & support non-real values for onnxFmu.cpp/.hpp

[StephanieKemna]

Changes need for onnxFmu.cpp and onnxFmu.hpp:

• There is an implicit conversion for onnxInputs from FMIReal to float, e.g. onnxFmu.cpp line 88 or line 108. The current arrays (e.g. onnxInputs_, onnxOutputs_, onnxStates_) are for float values instead of FMIReal. There are also comments in onnxFmu.cpp that it should be changed to handle non-real values.
• Nameless structs in onnxFmu.hpp create warnings.

Example warning:

..\src\mlfmu\fmu_build\templates\onnx_fmu\onnxFmu.cpp(88,38): warning C4244: '=': conversion from 'cppfmu::FMIReal' to '_Ty', possible loss of data