* interpolators use providers for client/situations/parts/ground elevation
* interpolators do no longer use own situations/parts, but those from provider
* interpolators are no longer QObjects (as it is not needed)
* use gnd flag from situation for gnd interpolation, there is no longer a parts gnd flag interpolation
* guess parts during interpolation
* changed iterators to m_s[i] as it makes clearer which values are used
** the flag is transferred from parts -> situation in airspace monitor
** if the other client already provides and gnd.flag in situation this also works
* adjusted logging
* use providers in unit tests / adjusted tests
* improved situation verification/assert
* this implementation is used by airspace monitor
* also used by dummy provider (advantage, unit tests test the real provider)
* some functions adjusted
* 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
