cyclic dependency between InstrumentItem and PointwiseAxisItem

Complications in GUI/Model are blocking progress on #71 (closed).

Class PointwiseAxisItem has member pointer m_instrumentItem, which is read by fcts PointwiseAxisItem::itemToAxis and PointwiseAxisItem::updateAxIndicators.

Fct PointwiseAxisItem::itemToAxis overrides BasicAxisItem::itemToAxis. Either of these is called from ScanningFunctionality::createScan or SpecularInstrumentItem::createCoordSystem, both in file InstrumentItems.cpp.

The call in SpecularInstrumentItem::createCoordSystem prevents us from simply passing the converter down.

Fct PointwiseAxisItem::updateAxIndicators is called (1) from InstrumentNotifier::notifyWavelengthDistributionChanged; that class has a m_instrumentItem;

and (2) from GrazingScanItem::initDiscreteAxis, which is called from ScanItem::updateToData, which is called from SpecularInstrumentItem::updateToRealData (part of the machinery is implemented for all types of scanning instruments, but actual synchronisation with external data is provided for specular instruments only).