Beam: replace most Beam() calls by static function horizontalBeam()

Core/Beam/Beam.cpp

Core/Beam/Beam.h

Core/Instrument/Instrument.cpp

GUI/coregui/Models/BeamItems.cpp

Tests/UnitTests/Core/Axes/DepthProbeConverterTest.cpp

Tests/UnitTests/Core/Axes/UnitConverter1DTest.cpp

Tests/UnitTests/Core/Detector/OffSpecularConverterTest.cpp

Tests/UnitTests/Core/Detector/RectangularConverterTest.cpp

Tests/UnitTests/Core/Detector/SphericalConverterTest.cpp

Tests/UnitTests/Core/Other/BeamTest.cpp

auto/Wrap/libBornAgainCore.py

auto/Wrap/libBornAgainCore_wrap.cpp