make fct static; use ASSERT to detect impossible cases

Sample/Scattering/IBornFF.cpp

Sample/Scattering/IBornFF.h

auto/Wrap/doxygenBase.i

auto/Wrap/doxygenCore.i

auto/Wrap/doxygenFit.i

auto/Wrap/doxygenParam.i

auto/Wrap/doxygenSample.i

auto/Wrap/libBornAgainBase.py

auto/Wrap/libBornAgainCore.py

auto/Wrap/libBornAgainFit.py

auto/Wrap/libBornAgainParam.py

auto/Wrap/libBornAgainSample.py