Functional Mock-up Interface (FMI) callback functions needed by
Java Native Access (JNA) so that a FMU * can perform functions like
allocating and freeing memory, printing log messages and handle the
end of a step.
If fmiDoStep() returns fmiPending, then
fmiGetStringStatus(..., fmiPendingStatus,...) can be called
to get the status of the asynchronously executing
fmiDoStep().