Adding functions to access dll handle of fmu instances

[Kuhnovic]

I ran into a situation where I needed to call a function that was not implemented according to the FMI standard, but was in the same DLL (a callback, something the FMI 2.0 standard does not support). This requires "raw" access to the underlying DLL or SO. I understand that this bypasses the FMI standard, but that's exactly the point: in the end an FMU is just a DLL, and the DLL can be more than just an FMU :)

[markaren]

I will get around looking into this eventually. Thanks for the PR

[markaren]

Thanks