clone returns IAxis; + IAxis::clipped; rm some unit tests

Base/Axis/Bin.cpp

Base/Axis/DiscreteAxis.cpp

Base/Axis/DiscreteAxis.h

Base/Axis/FixedBinAxis.cpp

Base/Axis/FixedBinAxis.h

Base/Axis/IAxis.cpp

Base/Axis/IAxis.h

Device/Data/Datafield.cpp

Device/Detector/IDetector.cpp

Sim/Export/PyFmt2.cpp

Tests/Unit/Device/DepthprobeConverterTest.cpp

Tests/Unit/Device/FixedBinAxisTest.cpp

Tests/Unit/Device/OffspecConverterTest.cpp

Tests/Unit/Device/PointwiseAxisTest.cpp

Tests/Unit/Device/RectangularConverterTest.cpp

Tests/Unit/Device/SphericalConverterTest.cpp

Tests/Unit/Device/SphericalDetectorTest.cpp

Tests/Unit/Sim/CoordSystem1DTest.cpp

auto/Wrap/libBornAgainBase.py

auto/Wrap/libBornAgainBase_wrap.cpp