Many examples cannot be imported to GUI

Many examples cannot be imported to GUI. Different failure modes are observed.

Assertion false failed in GUI/Model/FromCore/ItemizeSample.cpp, line 375

• scatter2d/CylindersAndPrisms.py !1806 (merged)
• scatter2d/MagneticCylinders1.py !1806 (merged)
• scatter2d/MagneticCylinders2.py !1806 (merged)

Assertion false failed in GUI/Model/FromCore/ItemizeSample.cpp, line 527

• scatter2d/CustomFormFactor.py
• scatter2d/FindPeaks.py
• scatter2d/GratingMc.py
• scatter2d/RectangularGrazing.py

Assertion false failed in GUI/Model/FromCore/ItemizeSample.cpp, line 173

• varia/MaterialProfileWithParticules.py

Popup error message at first attempt may be followed by segfault at second attempt (see also #643 (closed) for generic segfault problem). Scripts without "get_sample" section cause segfault at once (#645 (closed)).