fix compiler warnings

.../src/evidence-accumulation/ovexP300IEvidenceAccumulator.h

...ations/platform/P300Stimulator/src/ovexP300Visualiser.cpp

...ulator/src/properties/ovexP300InterfacePropertyReader.cpp

.../P300Stimulator/src/properties/ovexP300PropertyReader.cpp

...0Stimulator/src/properties/ovexP300ScreenLayoutReader.cpp

...ns/platform/P300Stimulator/src/visualisation/glGLabel.cpp

...s/platform/P300Stimulator/src/visualisation/glGSymbol.cpp

.../P300Stimulator/src/visualisation/glP300MainContainer.cpp

...al-processing/src/algorithms/basic/ovpCMatrixVariance.cpp

...sing/simple-visualisation/src/ovpCBoxAlgorithmErpPlot.cpp