add LambdaScan; fcts alpha_i and wavenumber depend on scan index

GUI/Model/Beam/SourceItems.cpp

Sim/Scan/AlphaScan.cpp

Sim/Scan/AlphaScan.h

Sim/Scan/IBeamScan.cpp

Sim/Scan/IBeamScan.h

Sim/Scan/LambdaScan.cpp

Sim/Scan/LambdaScan.h

Sim/Scan/QzScan.cpp

Sim/Scan/QzScan.h

Sim/Simulation/OffspecSimulation.cpp

Wrap/Swig/libBornAgainSim.i

auto/Examples/offspec/OffspecLambda.py

auto/MiniExamples/offspec/OffspecLambda.py

auto/Wrap/libBornAgainSim.py

auto/Wrap/libBornAgainSim_wrap.cpp

rawEx/offspec/OffspecLambda.py