rename all antennas to observers

applications/c8_air_shower.cpp

corsika/detail/modules/radio/CoREAS.inl

corsika/detail/modules/radio/RadioProcess.inl

corsika/detail/modules/radio/ZHS.inl

corsika/detail/modules/radio/detectors/AntennaCollection.inl

...ika/detail/modules/radio/detectors/ObserverCollection.inl

corsika/detail/modules/radio/observers/Observer.inl

...ika/detail/modules/radio/observers/TimeDomainObserver.inl

.../detail/modules/radio/propagators/DummyTestPropagator.inl

...ules/radio/propagators/NumericalIntegratingPropagator.inl

...s/radio/propagators/TabulatedFlatAtmospherePropagator.inl

corsika/modules/radio/CoREAS.hpp

corsika/modules/radio/RadioProcess.hpp

corsika/modules/radio/ZHS.hpp

corsika/modules/radio/detectors/ObserverCollection.hpp

corsika/modules/radio/observers/Observer.hpp

corsika/modules/radio/observers/TimeDomainObserver.hpp

corsika/modules/radio/propagators/DummyTestPropagator.hpp

...ules/radio/propagators/NumericalIntegratingPropagator.hpp

corsika/modules/radio/propagators/RadioPropagator.hpp