Add option to select alternative value unit for an extractor/DataIO

[ghorwin]

If an override value unit is set, the extractor should treat the data from the DataIO container as given in the alternative unit.

-> Requires change of code convertUnits() and in

Extractor2DOutputBase::axisUnitFromDataIO() and similar places.

Basically all code sections that use the dataio->m_valueUnit member variable need to be touched...