Leftover from JWu refactoring - to be reviewed after v22.

adapt scan to 1D datafile #974 (closed):

• review scan items #567 (closed) - !2691 (merged)
• deal with scaleFactor to rm GrazingScanItem - !2692 (merged)
• cover alpha, phi, lambda distributions by suite tests
• mv center/mean/median out of DistributionItem subclasses; rm Symmetric...
• rm initializer from PolyItem, only used with some scan items - !2686 (merged)
• mv readChosen, readItemFrom from PolyItem to XML; polymorphous arrays become independent of PolyItem - !2693 (merged)
• restore choice of scan type (q, alpha, lambda) in GUI
• support button to adapt scan to 1D datafile #974 (closed)

varia:

• reduce coupling between 1D and 2D data items

Other big topics

• Materials:
  • review materials
  • clarify linking between items
  • rm "id" field from serialization
  • get rid of Args... in PolyItem and UtilXML (only used for m_materials)
• Signalling
• Fitting and real-time simulation
  • automatize collection of real-valued parameters (currently ParameterTreeBuilder)