Added persistency for D_spectrum properties. This resolves #11 partially only, since there is no option to switch it off. I am not sure if that is needed though.

- Replace the std::reference_wrapper<TransferElement> with a TransferElementAbstractor.

- fixed last tests which were not using timeout based checks

separated DoocsSpectrum into header and cc file, as it should be. It was still header only from the time where it was a tempates class.