rename VariableBinAxis -> HistoAxis

Base/Axis/HistoAxis.cpp

Base/Axis/HistoAxis.h

Device/IO/DataFormatUtils.cpp

Tests/Unit/Device/IntensityDataFunctionsTest.cpp

Tests/Unit/Device/PowerfieldTest.cpp

Tests/Unit/Device/VariableBinAxisTest.cpp

Tests/Unit/Sim/CoordSystem1DTest.cpp

Tests/Unit/Sim/SpecularSimulationTest.cpp

Wrap/Swig/libBornAgainBase.i

auto/Wrap/libBornAgainBase.py

auto/Wrap/libBornAgainBase_wrap.cpp