improve SimDataPair data members and FitObjective API

• SimDataPair holds experimental data and their error estimates (uncertainties) in separate Datafields. This duplicates and shadows functionality from Datafield which also supports uncertainties. --- done in !1965 (merged).

• FitObjective::addSimulationAndData functions currently take raw arrays, then create Datafields. Thereby information on non-equidistant axes and axes labels is lost, then replaced by standard equidistant grid and labels "axis0|1". To be replaced by a function that takes a Datafield argument. => Almost done. Remainder is now #736 (closed) and #744 (closed).

All this is precondition for further work on #723 (closed).

Furthermore:

• No clue whether chi2 weighting with uncertainties was/is implemented correctly. Anyway, (I)ChiSquaredModule is badly overengineered. => this is now #743.