REFAC: Cleaning some Clang warnings

Modules/Filtering/Contrast/include/otbApplyGainFilter.h

Modules/Filtering/Contrast/include/otbApplyGainFilter.txx

Modules/Filtering/Contrast/include/otbBufferFilter.h

Modules/Filtering/Contrast/include/otbComputeGainLutFilter.h

...es/Filtering/Contrast/include/otbComputeGainLutFilter.txx

Modules/Filtering/Contrast/include/otbComputeHistoFilter.h

Modules/Filtering/Contrast/include/otbComputeHistoFilter.txx

Modules/Filtering/Contrast/test/otbApplyGainFilterNew.cxx

...es/Filtering/Contrast/test/otbComputeGainLutFilterNew.cxx

Modules/Filtering/Contrast/test/otbComputeHistoFilterNew.cxx