Throughout: use #error to indicate headers not exposed to Swig

Core/Computation/ComputationStatus.h

Core/Computation/DWBAComputation.h

Core/Computation/DWBASingleComputation.h

Core/Computation/DepthProbeComputation.h

Core/Computation/GISASSpecularComputation.h

Core/Computation/IComputation.h

Core/Computation/ParticleLayoutComputation.h

Core/Computation/RoughMultiLayerComputation.h

Core/Computation/SpecularComputation.h

Core/Element/DepthProbeElement.h

Core/Element/SpecularSimulationElement.h

Core/Export/ComponentKeyHandler.h

Core/Export/MaterialKeyHandler.h

Core/Export/NodeProgeny.h

Core/Export/PyFmt.h

Core/Export/PyFmt2.h

Core/Export/PyFmtLimits.h

Core/Export/SampleToPython.h

Core/Export/SimulationToPython.h

Core/Fitting/FitObserver.h