New methods in IAxis to get bin centers and boundaries.

Core/PythonAPI/src/AngleBinAxis.pypp.cpp

Core/PythonAPI/src/AxisBin.pypp.cpp

Core/PythonAPI/src/AxisDouble.pypp.cpp

Core/PythonAPI/src/FixedBinAxis.pypp.cpp

Core/PythonAPI/src/IAxis.pypp.cpp

Core/PythonAPI/src/VariableBinAxis.pypp.cpp

Core/Tools/inc/FixedBinAxis.h

Core/Tools/inc/IAxis.h

Core/Tools/src/FixedBinAxis.cpp

Tests/FunctionalTests/TestPyCore/axes.py

Tests/FunctionalTests/TestPyCore/intensitydata_io.py

Tests/UnitTests/TestCore/FixedBinAxisTest.h