Feat ref reducers #28

Implemented the following reducers and selector for the reference Settings and the correspondig tests for it. setActivePlotId, setYear, setModel, setVisibility, setOffsetApplied. for more detailed information please visit the Software Design Specification document. Also merged the actuall version of the ReferenceYearSlider branch and modified it concerning to the new reducers. I also merged it with a clone version of develop so that no or less merge conflicts will occur.