reorganisation of directories

See merge request !68

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.