RootObjectiveFunction adapter can work now with gradient functions.

Core/Fitting/FitSuiteKernel.cpp

Fit/Kernel/FitKernel.cpp

Fit/Kernel/FitKernel.h

Fit/Kernel/FitKernelImp.cpp

Fit/Kernel/FitKernelImp.h

Fit/Kernel/IMinimizer.h

Fit/Kernel/KernelTypes.h

Fit/Kernel/ObjectiveFunction.cpp

Fit/Kernel/ObjectiveFunction.h

Fit/RootAdapter/RootMinimizerAdapter.cpp

Fit/RootAdapter/RootMinimizerAdapter.h

Fit/RootAdapter/RootObjectiveFuncAdapter.cpp

Fit/RootAdapter/RootObjectiveFuncAdapter.h

auto/Wrap/libBornAgainFit.py

auto/Wrap/libBornAgainFit_wrap.cpp