Handling of "Use polarizer" flag is not consistent

In the UI, the user can define whether polarizer/analyzer functionality shall be used. This is stored in the flag bool InstrumentItem::withPolarizerAnalyzer() const.

This flag is not taken into account when creating the domain Instrument. See implementation in

std::unique_ptr<Instrument> Instrument2DItem::createInstrument() const

Just taking it into account in this instrument creation method will not do, since this flag is not set when creating a GUI InstrumentItem from a domain Instrument.

Current impact: If a user works with a polarizer/analyzer setting, then decides to uncheck the flag in the UI and tries to run a simulation, his simulation will be wrong.