Move trivial getters from cpp to h files

For uniformity of style.

Search for \{\s*(return m_\w+;)\s*\} in *.cpp returns

Param/Distrib/DistributionHandler.cpp return m_distributions;
Resample/Processed/ReSample.cpp return m_stack;
Resample/Processed/ReSample.cpp return m_relayouts;
Fit/Param/Parameters.cpp return m_corr_matrix;
Fit/Param/AttLimits.cpp return m_fixed;
Fit/Option/MultiOption.cpp return m_value;
Fit/Option/MultiOption.cpp return m_default_value;
Sim/Export/MaterialKeyHandler.cpp return m_Key2Mat;
GUI/Model/Material/MaterialItem.cpp return m_delta;
GUI/Model/Material/MaterialItem.cpp return m_beta;
GUI/Model/Material/MaterialItem.cpp return m_sld_re;
GUI/Model/Material/MaterialItem.cpp return m_sld_im;
GUI/View/FitControl/GUIFitObserver.cpp return m_block_update_plots;
GUI/View/FitControl/FitWorkerLauncher.cpp return m_fit_start;
GUI/View/FitControl/FitWorkerLauncher.cpp return m_fit_end;
GUI/View/FitMessage/FitLog.cpp return m_messages;
GUI/View/Overlay/FullframeOverlay.cpp return m_item;
GUI/View/Overlay/LineOverlays.cpp return m_item;
GUI/View/Overlay/PolygonOverlay.cpp return m_item;
GUI/View/Overlay/EllipseOverlay.cpp return m_item;
GUI/View/Overlay/VertexOverlay.cpp return m_item;
GUI/View/Overlay/RectangleOverlay.cpp return m_item;
Base/Py/ArrayWrapper.cpp return m_datasize;
Base/Py/ArrayWrapper.cpp return m_array_ptr;
Base/Py/ArrayWrapper.cpp return m_owndata;
Device/Data/Datafield.cpp return m_err_sigmas;
Device/Data/Datafield.cpp return m_values;