Provide default values in Python API version of MaterialProfile

Core/Computation/ProfileHelper.cpp

Core/Computation/ProfileHelper.h

Core/Multilayer/MultiLayerFuncs.cpp

Core/Multilayer/MultiLayerFuncs.h

...s/python/simulation/ex06_Reflectometry/MaterialProfile.py

Wrap/swig/extendCore.i

Wrap/swig/libBornAgainCore.i

Wrap/swig/renameCore.i

auto/Wrap/libBornAgainCore.py

auto/Wrap/libBornAgainCore_wrap.cpp