Refactor model: BeamItems

See merge request !67

Refactor model2: improve interface of IntensityDataItem

See merge request !66

Refactor Distribution Handling

See merge request !65

This is not the same behavior as before, since the limits have not been checked by registerParameter. But since the limits are not checked later on (no Parameter->setValue(d) existing any more), it seems reasonable to introduce this check on initialization.

Refactor model: DataItem and SpecularDataItem

See merge request !64

disable the tests so that CTest lists them as "disabled", instead of just keeping them away from CTest

fix values of FTDecayFunctionVoigt 1D & 2D

See merge request !62