ENH: metadata parsing for DIMAP products

Modules/Core/Metadata/include/otbDimapMetadataHelper.h

...Core/Metadata/include/otbFormosatImageMetadataInterface.h

...Core/Metadata/include/otbPleiadesImageMetadataInterface.h

...les/Core/Metadata/include/otbSpotImageMetadataInterface.h

...s/Core/Metadata/src/otbFormosatImageMetadataInterface.cxx

Modules/Core/Metadata/src/otbMetaDataKey.cxx

...s/Core/Metadata/src/otbPleiadesImageMetadataInterface.cxx

Modules/Core/Metadata/src/otbSpot6ImageMetadataInterface.cxx

Modules/Core/Metadata/src/otbSpotImageMetadataInterface.cxx