[Mat] Employed fixture for test and improved the documentation

...Lib/PorousMedium/Permeability/createPermeabilityModel.cpp

...alLib/PorousMedium/Permeability/createPermeabilityModel.h

MaterialLib/PorousMedium/Porosity/ConstantPorosity.h

MaterialLib/PorousMedium/Porosity/Porosity.h

MaterialLib/PorousMedium/Porosity/createPorosityModel.cpp

MaterialLib/PorousMedium/Porosity/createPorosityModel.h

MaterialLib/PorousMedium/PorousPropertyHeaders.h

MaterialLib/PorousMedium/Storage/ConstantStorage.h

MaterialLib/PorousMedium/Storage/Storage.h

MaterialLib/PorousMedium/Storage/createStorageModel.cpp

MaterialLib/PorousMedium/Storage/createStorageModel.h

Tests/MaterialLib/TestPermeabilityModel.cpp

Tests/MaterialLib/TestPorousMediumPorosity.cpp

Tests/MaterialLib/TestPorousMediumStorage.cpp