Instead of requesting it individually for each callsign and waiting for a
signal from xswiftbus, pass in a list of callsigns and use async callback
to read the data.
* return interpolation results parts/situation in one step
* base class init for each interpolation step, less redundant code
* removed old interpolation/parts function
* adjusted unit tests and simulator objects (such as SimObject)
* no longer using the add parts/situations signals
* will use the remote aircraft provider parts directly
Result: all parts/situations are stored only once in memory and in one single place
