CI: Use QT_QPA_PLATFORM=offscreen

BUG: MANTIS-791 (Accuracy error when using 'Dimensionality reduction'); Missing parenthesis in Scott's forumla used to compute number of bins of histogram lead to a std::bad_alloc.

ENH: Wrote histogram file header; Added stream operators for overall types (itk::VariableLengthVector<> and itk::Statistics::Histogram<>).

ENH: Improved VectorImageModel::Settings interface; Optimzzed modified and applied flags of ::Settings (not setting flag when not needed).

ENH: Added mvd::ToStdString() and mvd::FromStdString(); Replaced bad conversions by safe To/FromStdString() functions.

COMP: remove call of QString::fromStdString since it causes runtime issues on windows, when Qt and Mvd are not built with same VC version

ENH: Fixed unecessary virtual instead of inline methods; Added otb::ToFixedArray() and otb::ToVariableLangthArray() rebinders (mvdAlgorithm); Fixed ColorDynamicsWidget default layout; Fixed HistogramModel access to Min/Max pixels.

ENH: Added ColorSetupWidget::Set/GetCurrentIndex(); Documented ColorSetupWidget; Initialized ColorSetupWidget and VectorImageModel::Settings with ImageMetadataInterface::GetDefaultDisplay() on File/Load.

REFAC: Changed order of includes (ConfigureMonteverdi2.h before Qt before other includes to prevent system compilation problems).

ENH: Implemented basic version of Model (AbstractModel<-AbstractImageModel->VectorImageModel); loading on File/Open and storing in Application.

REFAC: Updated MyClass skeleton file (Qt 'TRANSLATOR: mvd::MyClass' support for C++ namespaces to comply with Qt lupdate translation tool needs).